The main characteristic of the spatial distribution of geomagnetic low-point time is that it changes with longitude. To solve the analytical formula of the relationship, we first calculated the low-point time sequences and the expected values of the low-point times of 13 geomagnetic stations in Gansu Province. Using nearly three years of data from the national geomagnetic network, the frequency distribution diagram and the normal density curve of the low-point time series were plotted. We then made a linear regression for the relationship between the low-point time of the Z component and longitude of the station, and indicated that the linear regression equation should be infinitely close to the "Greenwich time-longitude" equation. In this paper, the relational expression between the low-point time of the Z component and longitude of the station is first proposed, and the approximation equation of the relational expression is described. The expected value of the low-point time of each station is the normal background value of the geomagnetic low-point time, which is a reference standard to determine whether the geomagnetic low-point time is abnormal. The significance of the above work is to perfect the geomagnetic low-point displacement method proposed by Ding Jianhai.