In order to calculate the burnt mass fraction of complex three-dimensional (3D) propellant grains to meet the requirements of interior ballistic modelling; the level set method is introduced to emulate and calculate the burning surface area of partially cut 7-perforated propellant. The surface evolution and simulation of a 3D grain of partially cut 7-perforated propellant are divided into two parts: the grain configuration initialization and the level set calculation of the propellant regression process. Parallel layer burning is assumed so that the burning surface regresses layer by layer in a direction normal to the surface until the grain is burnt completely. As the burnt mass fraction increases, the remaining propellant volume decreases gradually. The level set method easily simulates the slivering process for complex grain geometries. In this way, the burnt mass fraction of partially cut 7-perforated propellant grain can be calculated by the level set method for the entire combustion process. Results show that the level set method is suitable to capture the burning surface for each burning step and its related parameters, such as the burning area, the remaining propellant volume and burnt mass fraction. More importantly, the level set method gives a possible solution to the coupling of grain combustion with the internal fluid simulation by the pressure and velocity. It is impossible for geometry-based methods to integrate the internal fluid parameters in an interior ballistic model. Also, the level set method will benefit substantially the grain design and lead to improved internal ballistic performance.