With the use of ultra high-performance concrete (UHPC) in frozen areas, UHPC structures suffer from severe erosion under the interaction of salt ions and frost, resulting in significant performance degradation of UHPC structures. However, the performance degradation mechanism of different compositions and structures of UHPC under the coupling erosion of salt and frost is still unclear at present. In order to study the degraded behavior and mechanism of UHPC under the coupling erosion of salt and frost, various UHPCs were prepared and exposed to freeze–thaw (F-T) cycles and salt–freeze–thaw (S-F-T) cycles. The mass and compressive strength of the UHPCs before and after the F-T and S-F-T tests were compared to evaluate the performance degradation of UHPCs. The SPSS software correlation method, scanning electron microscope, and mercury intrusion porosimetry tests were used to analyze the mechanism of the coupling erosion of salt and frost on the UHPCs. Results indicate that UHPC with a lower water–binder ratio (WBR) and higher superplasticizer content exhibits better resistance to the coupling erosion of salt and frost. The resistance of UHPC to the coupling erosion of salt and frost will decline with the rise of WBR, and the WBR is the most significant factor affecting the coupling erosion of salt and frost for UHPC. The coupling of salt and frost decreases the compressive strength and leads to cracking of UHPC, which can be ascribed to the larger average pore size and higher cumulative pore volume caused by the salt ions. The hygroscopicity of salt ions increases the moisture in pores, and salt crystallization generates pressure in the pores, further aggravating the damage of UHPC.