London — Prime Minister Margaret Thatcher's belief in strong government is ensuring her second administration is at least as controversial as her first. Some of her Conservative Party supporters are growing thoughtful about the political implications of her assertive style.
At present her determination to be ''the strongest man in my Cabinet'' is being displayed in three areas:
* She has stuck to her view that the European Community must reform its own finances and give Britain a better membership deal. This placed her in a minority of one at the Brussels EC summit which collapsed, her detractors say, because of Mrs. Thatcher's obduracy.
* Faced with a rolling strike by British coal miners, she has ordered police in many coalfields to try to prevent so-called ''flying pickets'' impeding miners who wish to work from doing so. Opposition Labour Party critics have accused her of authoritarianism, because use of police has prevented the free movement of miners from one county to another.
* To stem the leaking of government secrets to the press, the prime minister is using old statutes to prosecute the leakers. A young Foreign Office worker who gave the Guardian newspaper a memorandum about secret government moves to arrange the arrival of cruise missiles in Britain last December has been sent to prison for six months by an Old Bailey judge.
The case of Sarah Tisdall promises to become a legal and political cause celebre. She received an exemplary sentence, but others earlier accused of more serious charges were fined or set free. She is appealing the sentence.
Opposition leader Neil Kinnock said that in pressing charges against a person who had not been a spy and had not handed over information that could help an enemy, the government had shown ''the mixture of malice and weakness that is characteristic of all bullies.''
Miss Tisdall was accused under Section 2 of the Official Secrets Act, a law passed in 1911 and widely regarded by parliamentarians as obsolete and dangerously flexible in its terms.
A sign of growing Conservative backbench concern about Mrs. Thatcher's style came swiftly after the Brussels summit collapsed. She returned to London speaking of possibly withholding payments from the EC until the budgetary issue was settled. Backbenchers quietly reminded her such action would be illegal.
Even more striking was parliamentary reaction to the use of saturation police tactics to prevent Yorkshire miners who supported the coal strike from traveling in flying pickets to other coalfields.
Opposition MPs and some Conserva-tives made it clear that freedom of movement was a key civil right which the government was curbing. The National Council for Civil Liberties attacked the way police were being used.
Mrs. Thatcher's supporters claimed the president of the National Union of Mineworkers was himself flouting the law by condoning the use of flying pickets and refusing to condemn their violent tactics. Something had to be done to redress the balance. That something involved spending an extra (STR)5 million ($ 7.2 million) in one week to deploy police to trouble spots.
Underlying the debate about Mrs. Thatcher's governing style is a growing belief that, despite their large majority in Parliament, the Tories are being unnecessarily authoritarian.
Mr. Kinnock claims the root cause is the prime minister's determination to be boss. He argues that this is producing bitterness and intolerance that are seeping into other areas of British life.