The effects of heat treatment on microstructure and creep properties of β high-strength titanium alloy, Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe, were studied. After solution treatment at 790 °C and aging treatment (HT1), the microstructure is composed of equiaxed α_p phase, β phase, α_s phase, and becomes β phase and α_s phases after solution treatment at 840 °C and aging treatment (HT2). The creep behavior at 400 °C was analyzed. The stress exponents of both alloys are between 1 and 2, indicating that the diffusional creep mechanism is one of the dominant creep mechanisms. The alloy after HT2 treatment has better creep resistance and a subsequent creep test on this alloy was performed at 450 °C under 400 MPa. The creep fracture has the mixed ductile-brittle characteristics. The phase interfaces can hinder the dislocation movement, and the α_s phase can coordinate with the matrix to deform, thereby reducing the occurrence of intragranular cracks.