The invention discloses a software test case automatic generating method, belonging to the technical field of software test automation. The method comprises the following steps: 1, in the first phase, establishing a component Petri network model on the basis of a tested object, wherein three objects, namely a structured language program, requirement or scene based description and a object oriented program are taken in the first phase); 2, in the second phase, collecting and storing fore-set and post-set of places and changes in a Petri net (PN) model and related information of initialized condition and other model structures by utilizing an appropriate data structure; 3, in the third phase, analyzing and verifying the model (deadlock, activity, boundedness, accessibility and the like) by adopting a proper algorithm according to the information collected in the second phase; 4, in the fourth phase, generating a test case; 5, in the fifth phase, analyzing the test case according to different coverage rate indexes; and 6, in the sixth phase, comprehensively analyzing according to the original requirement and quality requirement of a user so as to re-generate a test case set in accordance with the requirements.