The pyrolysis of three oil samples from Tarim Basin was carried out under 390°C and high pressures formed by injecting N2 in the closed system and the influences of temperature, time and the oil compositions on the pyrolysis reaction of oil to gas were discussed. The gas generation ratio increased and average activation energy decreased with the content of polar compositions in oil, and the gas generation ratio changed intensively with reaction time for high content of polar compositions under the experimental conditions. The effect of pressure on pyrolysis of oil was tightly related to oil composition. High pressure was beneficial to the pryolysis of oil with more polar compositions. The calculated value of the gas generation ratio fitted well with experiment value as the parallel first order reaction model containing 11 reactions with different activation energies and different preexponential factors was used for simulating the pyrolysis reaction of oil to gas by minimizing objective function with MATLAB optimization module.