Abstract: Steel fiber can reinforce the compressive strength of ultra-high performance fiber reinforced concrete (UHPFRC), but it will reduce the flowability of fresh UHPFRC, thus weaken this reinforcement effect and the work performance of UHPFRC. In order to study this adverse effect, two sets of tests A and B were carried out with steel fiber volume fraction and aspect ratio as variables. In the A series test, the fixed water-to-binder ratio was 0.18 and didn’t control the flowability. It mainly studied the effect of steel fiber on flowability and compressive strength. The test results show that the flowability of fresh UHPFRC decreases with the increase of the volume fraction or aspect ratio of steel fiber; when the volume fraction of steel fiber exceeds a certain value (2.00vol%), the expansion degree decreases significantly, and the corresponding compressive strength enhancement effect also decreases. The X-ray CT scan test finds that the incorporation of steel fiber weaken the self-compacting ability of the fresh slurry, increasing in the internal pore size and porosity of the hardened matrix, thereby weakening the compressive strength. Comprehensively considering the positive and negative effects of steel fiber incorporation into compressive strength, a semi-empirical prediction formula for compressive strength was proposed. In the B series test, the water-binder ratio was changed and the expansion degree was controlled to 240 mm. Through comparative analysis of the A series test results, the effect of the volume fraction and aspect ratio of the steel fiber on the compressive strength after the control of the flowability was studied. The results show that when the volume fraction of steel fiber is large, the water-binder ratio should be increased to ensure a certain flowability, which can effectively improve the reinforcement effect of the fiber; when the volume fraction of fiber is small, the water-binder ratio should be decreased on the premise of meeting the flowability requirements, which can further improve the compressive strength. In the design of the UHPFRC mix ratio, in order to improve the reinforcing effect of steel fiber and ensure that UHPFRC has good working performance, the adverse effect of incorporating steel fiber on the flowability of UHPFRC should be considered.