It is considered commonly that the sealing capability of muddy cap rock becomes worse coupled with its deepening diagenesis. However, a lot of hydrocarbon discoveries and test results proof that muddy cap rock at high evolutionary stage has high breakthrough pressure and good sealing capability .What is the controlling factors affect the sealing capability of muddy cap rock and How to determine the developmental boundary condition of cracks in muddy cap rock are basic problems that constraint the effectiveness evaluation of cap rock. According to fluid inclusion thermometry in veins existed in mudstone, combined with restore of buried history, the formation time of veins in different layers have been determined. The results show that younger cap rocks break firstly, and older cap rocks break latterly. The results proof that the development of cracks in muddy cap rock have relation with uplift magnitude. The rocks will break at certain depth. The breaking process of mudstone in the process of uplift has been molded using triaxial stress test, the results of which show that mudstone at high evolutionary stage has plasticity property and good sealing capacity in some buried depth. The development of cracks in cap rocks has relation with buried depth and change of maximum main stress difference.