The invention relates to a high temperature vacuum wind tunnel test cabin with the sliding thermal compensation function, a diffuser integral structure and a sliding thermal compensation method, solves disadvantages in the prior art, provides a thermal compensation scheme of a high temperature wind tunnel diffuser and solves a problem of thermal compensation difficulty of a large-scale gas flow wind tunnel test cabin under long-time, high temperature and large thermal flow bad thermal environments. A structure design of the diffuser of which the middle segment is fixed and supported and two ends are in a sliding mode is employed, and thermal expansion is reasonably distributed. According to the different thermal environments, different thermal compensation modes are employed at different positions. For the upstream, the contact area between a cabin body and the diffuser is enhanced through adding a conic sleeve, multi-channel sealing is employed to eliminate being-burnt and sealing failure risks, an upstream end of the diffuser, namely a collection port, is a free end, and sliding thermal compensation is realized; for the downstream, after a water spraying temperature reduction device is employed to reduce the gas temperature, an expansion knot thermal compensation method is employed, and sliding thermal compensation is realized.