Abstract: The focal spot size of X-ray tube is one of the key factors affecting the imaging resolution, and it is also an important index to characterize the working performance of the X-ray source. Typically, after the X-ray tube is manufactured, the focal spot size must be measured and marked. Nowadays, the most widely used measurement standards for X-ray tube focal spot size in the world are IEC 60336, EN 12543, and ASTM E1165. Between various standards, there are differences in the focal spot size measurement methods, scope of application, loading factors and imaging equipment. Among these, the slit method of EN 12543 standard is mostly utilized in industrial X-ray tubes. However, due to its complex and precise design requirements, the test equipment, i.e., the slit camera, is difficult to produce and process, and the cost is high, which is not conducive to promotion and application. This paper studied the influence and variation of the measurement of the focal spot size at different slit sizes by using double tungsten alloy cylinders instead of slit camera. Based on the calibration of distance and position parameters in the cylinder method, the measurement equipment was conveniently positioned, and the distance between the focal spot and the translation mechanism was measured simultaneously, which improves the position accuracy of the focal spot size measurement. Additionally, the formula for determining the focal spot size in the slit method was improved, so that the slit method can be applied to a wide range of slit widths. In particular, the geometric magnification of the system can be measured directly when the slit width is large, skipping the step of measuring the distance between the focal spot and the translation mechanism. The experiment scheme has low requirements on the centering conditions of the equipment and the distribution of the X-ray source. In the data processing process, dark field correction and bright field correction were used to eliminate the problem of inconsistency in the response characteristics of each detector pixel, and the Wiener Filtering was performed on the signal output by the detector to deal with Gaussian noise and uniformly distributed noise. The experiment results show that relatively accurate measurement values can be obtained by measuring MXR-225HP/11 with double tungsten alloy cylinders with the same spacing as the focal spot size. The measurement relative error is less than 10%. According to international standards, when the measurement results are within plus or minus 10% of the theoretical value, it can be used as a method to determine the focal spot size. Therefore, the improved method in this paper is reliable and does not require high testing equipment, allowing it to be widely used. In addition, the geometric magnification of the focus measurement device can be directly calculated when the slit width is larger than the focal spot size, and the calculation results are consistent with the results measured in the cylinder method.