The intrinsic parameters of the camera are mainly composed of the focal length, coordinate of the main point and the tilting factor. The focal length is an important parameter for the camera to obtain the three-dimensional information from the two-dimensional image, which determines the shooting angle of the lens. It can be obtained by the camera calibration, such as the Zhang's calibration method, but the process is tedious and takes a long time. In this paper, a fast method to extract focal length of camera is proposed. To improve and simplify the camera self-calibration algorithm, parallel particle swarm optimization (PSO) algorithm is applied to solve the improved Kruppa equation, making the pole computation converted to the fitness function of cost function. In addition, in the process of PSO calculation, parallel computing experiments are carried out on the multi-core computer platform, and the advantages of parallel computing are fully used. The experimental results show that the precision of the method's focal length is close to Zhang's calibration method, and the time of the algorithm is greatly reduced compared with other calibration algorithms.