Abstract: The law of crack propagation was analyzed through uniaxial compression test of plate shale,containing two colinear prefabricated closed collinear fractures. From the perspective of fracture mechanics,the crack initiation se- quence was explained,and the brittle fracture criterion and compressive strength of plate shale were derived,and the calculation method of fracture toughness was given. The results indicate that The deterioration degree of compressive strength of pre-cracked shale is larger than its elastic modulus. After the damage of the rock,there emerge two classes of macroscopic cracks,wherein one is tensile crack being flat and smooth,the other one is compression-shear crack which is jagged and has obvious indentation. Cracks are more likely to be initiated at the inner end of prefabricated fracture than initiated at its outer end,whose initial angle ranges from 46°to 86°. The smaller the space between frac- tures it is,the easier the rock bridge can be formed through crack coalescence. The initial direction is only relevant to θ. The reason why cracks are more likely to be initiated at the inner end of prefabricated fracture is that the stress in- tensity factor at the inner end is bigger than outer end. The advantageous crack angle of collinear crack model is βm = 0. 5 arctan f-1 .