To study the size effect of mechanical properties of steel-polypropylene hybrid fiber ultra-high performance concrete（HF-UHPC）， the cube compressive strength and axial compression mechanical properties of HF-UHPC specimens with different sizes were studied by considering the influence of fiber parameters. The results show that the size effect of cube compressive strength， axial compressive strength and axial compression peak strain of the specimen becomes more significant with the increase of steel fiber content and length-to-diameter ratio. With the increase of polypropylene fiber content， the size effect of cube compressive strength， axial compressive strength and axial compression peak strain of the specimen changes slightly， showing a trend of weakening at first and then increasing； The size effect of elastic modulus is little influenced by the parameters of hybrid fibers， and it can be ignored. In addition， based on the experimental results， the mechanism of size effect of compressive strength of the specimen is revealed， and the calculation formula of size effect law parameters of compressive strength of the specimen is established， which can be used to calculate compressive strength of the specimen with different sizes.