The development of optical parametric chirp pulse amplification (OPCPA) numerical simulation platform involves physical models such as broadening and compression of optical pulse, parametric amplification and focusing output. In the simulation platform, the Fresnel far-field diffraction equation is usually used to simulate the characteristics of ultrashort pulse focusing. Firstly, we need to calculate the optical field distribution of different wavelength components in the ultrashort pulse, and then use the inverse Fourier transform to obtain the temporal and spatial distribution characteristics of the pulse. However, for different wavelength components, the sizes of focused field grids obtained by the far-field algorithm are not equal, and subsequent resampling is required, which will increase the amount of calculation. In addition, due to the limitation of the calculation range of the light field in the pulse broadening and compression, there is also a problem of poor resolution of the focused field. In this work, the mathematical expression that can adjust the range of the output light field and use the fast fourier algorithm is derived. The main mechanism of this algorithm is as follows. Based on the Fresnel far-field diffraction equation, the output field is sampled independently in the discrete calculation process to meet the requirements for adjustable range of the output field. After identity transformation, the output field results can be calculated by the fast Fourier algorithm. Furthermore, the sampling conditions that need to be satisfied when using the algorithm are further analyzed and discussed. It solves the problem of how to improve the resolution of light field and keep the computational grid size of each wavelength component consistent when the traditional Fresnel far field diffraction is used to simulate the focusing process, which provides the convenience for the subsequent direct time-frequency inverse transformation. The numerical simulation results reveal that the dark ring region of the ultrashort pulse focusing field shows strong spatiotemporal coupling characteristics. This algorithm has been successfully applied to the development of OPCPA numerical simulation platform, and is expected to play an important role in optimizing the design of ultrashort laser pulse device.