The utility model provides a high-temperature high-pressure volume expansion contraction detecting instrument for oil well cement, which belongs to the field of devices for detecting expansion and contraction of the volume for the oil well cement. The detecting instrument includes a high-temperature high-pressure reaction kettle, a temperature control system, a pressure control system, a controller and a computer data processing system. The detecting instrument can simulate the expansion rate and the contraction rate of the volume of the oil well cement in a vertical shaft or horizontal well under maintenance temperature and pressure and record temperature pressure curves and time-and-expansion-rate curves. The detecting instrument is adjustable in experimental pressure, automatic in control of experimental temperature and capable of displaying the experimental pressure and temperature in real time. The curves of the volume expansion and contraction curves of cement slurry can also be displayed in real time. The experiment is simple in operation and capable of automatically outputting and storing experimental results and generating an experimental report. The detecting instrument is reasonable in structure arrangement and concise in appearance.