Based on analysis and the study on No.54 item of "Coal Mine Safety Regulations",several failure forms of supported object in coal working face are pointed out and their corresponding failure models also are built.On the base of the failure models and theoretic analysis,new ideas of the minimum supporting strength and the maximum supporting strength are brought forward,and the decisive factors of the values of the two supporting strengths are presented.Also the new ideas and the results of theoretic calculation are proved by in-situ measurement and experimentation in some two top coal caving mining faces,and the initial safe supporting strength in coal mining face be concluded.