Getting real cylinder pressure is the basis of engine combustion analysis. Because of the advantages of good thermal performance, fast response, small size, high accuracy, large range and so on, piezoelectric quartz sensor is widely used in the measurement of the cylinder pressure. But this kind of sensor can only get the dynamic cylinder pressure which may not represent the real one. In this situation, the cylinder pressure needs to be corrected by some method. It also could cause great result divergences of the combustion analysis by using different cylinder pressure correction methods. This paper aims to acquire a proper cylinder pressure correction method by carrying out the theory analysis based on ideal gas equation and experiment research of cylinder pressure on a turbocharged eight-cylinder diesel engine. In the premise of the ignorance of the Air leakage of cylinder, The internal charge temperature of the cylinder is close to the wall temperature in some range of consecutive crank angles, that means the heat exchange between the cylinder charge and the wall is equivalent to zero. In this range of consecutive crank angles, the internal charge of the cylinder is truly adiabatic and the value of the polytropic exponent should be a constant. It can get a nearly fixed polytropic exponent in this range of consecutive crank angles by the monolithic translation of the measured cylinder pressure. From this point, This method could get real polytropic exponent and correct cylinder pressure curve at the same time. The further experiment research on this engine shows the value of polytropic exponent fluctuates from 1.33 to1.38 under different conditions, and has little relation with the engine load, but it would become bigger with the increase of the engine speed.