The residual mechanical properties and microstructure of UHPC mixed with 6, 8, 12 mm copolymer formaldehyde fiber and steel fiber after high temperature were studied. The test results show that at 400 ℃, only plain concrete bursts. At 500 ℃, the steel fiber UHPC specimen burst, while the UHPC specimen with single copolymer formaldehyde fiber and mixed with two kinds of fiber can still maintain relatively integrity, and the latter has higher residual strength at high temperature. SEM and pore structure show that the copolymer formaldehyde fiber in UHPC can be closely bonded to the matrix after steam curing at high temperature. The pore formed by fiber melting at 165 ℃ can effectively relieve and release steam pressure, keep the concrete matrix intact and prevent it from cracking at 500 ℃. The research results reveal the mechanism of the copolymer formaldehyde fiber on the high temperature performance of concrete, which has great reference value for its application in UHPC.