Instrument background is an important content in implementing the space-based astronomical mission. For the focusing X-ray telescope, the observation ability is affected by the particle background, which is directly related to the sensitivity of the instrument and the systematic error of background reproducibility. In the iterative process of instrument design and engineering implementation, it is necessary to make sure that the particle background level is within the acceptable level. In this paper, we propose a method of fast estimating the particle background of the space-based focusing X-ray telescope, which is based on interpolation of planar density distribution. With acceptable accuracy and efficiency, this method is suitable for rapidly estimating the background shielding effects of various design schemes, especially in the early stage of telescope scheme design. This can greatly improve the availability of early scheme design. This method has a certain reference significance for developing the focusing space high-energy astronomical instruments and other similar instruments. The commonly used method of estimating the particle background of space X-ray instruments is the Monte Carlo method, which relies on constructing an overall mass model of instrument and simulating the response of the detectors to the space radiation environment, but the calculation efficiency of this method is lower. In order to meet the needs of instrument design optimization of mission during initial stage, we simulate the responses of simplified aluminum spherical shells with different sizes and planar desities to the space radiation environment, and count energy depositing events in a concerned energy range. Then we obtain the relationship between the particle background caused by various spatial radiation components and the thickness of the simplified aluminum spherical shell after being normalized. The particle track tracking method is used to calculate the area density distribution of the equivalent aluminum around the sensitive detectors of the telescope. Finally, the average particle background level of each component is obtained by interpolating calculation according to the relationship between equivalent thickness and the particle background. The method is verified through the simulation of the payload SFA onboard eXTP satellite by comparing the results of the simulation calculation of the whole star mass model with the results from the area density distribution interpolation method, and good consistency is obtained. The method based on the interpolation of the planar density distribution can well depict the relationship between the whole structure and the particle background level, which can be applied to the particle background estimation and shielding optimization for X-ray focusing instruments in different orbital space radiation environments.