The mass fraction burned (MFB) of the gas mixture in combustion process as the function of the combustion duration time, reflects the percentage of fuel consumed at the every moment in the combustion process, and is an important factor for analysing of the fuel combustion performance, and it has high engineering value for evaluating engine design parameters and operating performance and for improving the combustion process in cylinder combustion process. The ion current signal directly originated from the fuel combustion process in the cylinder, and contained much information about the combustion process; the MFB of fuel in combustion process can be obtained based on the correlation between the MFB and ion current signal. In this paper, the ion current method is constructed to calculate the MFB of fuel in combustion process under different initial conditions (initial pressure from 1 bar to 3 bar, equivalence ratio from 0.8 to 1.2, hydrogen mass fraction from 0% to 10%, and CO2 dilution ratio from 0% to 10%), the method based on the waveform parameters of ion current signal for obtaining the efficiency factor and the form factor of Wiebe function, and further to slove the mass fraction burned of combustion process. The mass fraction burned is obtained through ion current method reduced assumptions error and improved the calculation accuracy compared with the well-known Rassweiler-Withrow method, and the ion current method has an advantage that not requires an high-cost sensor and some rather complex measuring instruments. The ion current method is a simple but powerful calculation model that provide a new technical path for analysing and online obtaining the MFB of fuel in combustion process. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.