To calibrate the equivalent focal length of a fish-eye camera is a co mplicated problem.This paper puts forward a three-point calibration method to solve the challenging situation.Based on the equidistant projection model and the spatial model of a fish-eye camera,consid ering the cosine theorem,this paper derives a theoretic equation,only containing one unknown quantity of equivalent focal length, which lays a solid foundation for measuring the equivalent focal length of a fis h-eye camera by the three-point method.To verify the three-point calibration method experimentall y,this paper conducts a simulation test in practice.Randomly select three coinciding points on the grou nd,measure the distance among them and then calculate the equivalent focal length by the fish-eye camer a image which contains the three points.The three-point calibration method simplifies the procedure an d reduces the complexity of measuring a fish-eye camera equivalent focal length.The results of the simul ation show the accuracy and the feasibility of the three-point method.The equi valent focal length calibrated by using the three-point method is more close to the fac tory parameter of fish-eye lens than other traditional methods and toolbox.