The B and C annulus trapped pressure cannot be released through the wellhead due to the special structure of the underwater wellhead. Foam spacer can effectively reduce Annular Pressure Build-up( APB) in deep water wells. At present, there is nocresearch on the theoretical model and experimental verification of foam spacer, and hard to guide the site foam spacer injection operation. Based on the equation of state of gas and liquid, considering the influence of radial deformation of casing, the calculation model of APB is established. The thermos-physical parameters of the fluid were measured by laboratory test. The best gas-liquid ratio of foam spacer injection in deepwater wells was analyzed by laboratory test and theoretical analysis. It can be seen from the analysis that the thermal expansion coefficient and compression coefficient of annulus fluid change with the temperature and pressure. When the volume of annulus gas increases, the annulus trapped pressure decreases. The pressure tends to be stable when the gas volume ratio in annular reaches 5% . The analysis results have theoretical guiding for the prevention and control of annular trapped pressure in deep water wells.