Archie's formula is the foundation of sandstone reservoir using well logging data to calculate oil saturation. But in the mud, low porosity and low resistivity reservoirs, the effect becomes worse. The main reason is that the applicable conditions of the Archie formula lacks clear understanding. Based on a large number of experimental data of rock electricity, this paper uses the method of function analysis to analyze the applicable conditions of Archie formula. The results show that the applicable condition of Archie formula is implied: under the same lithology and different pore rock, the rock electrical parameters a, b, m, n value should be exactly the same or approximate; electrical parameters of rocks with different sand a, b, m, n value, the greater the gap, the worse effect of using Archie formula. At the same time, the function formula of Archie formula can be well applied in shaly sandstone, low resistivity reservoir or complex pore reservoir. The difference is that the rock electrical parameters have obvious change compared with conventional sandstone reservoir. Because there is no better method to determine the a, b, m and n values of rock electrical parameters by point or by layer in logging interpretation, the unreliability of rock electrical parameters in interpretation is an important reason affecting the calculation accuracy of oil saturation. Therefore, it is an effective way to improve the coincidence rate of logging interpretation by using the corresponding rock electrical parameters in different layers of sandstone. The study points out the key problems of the current use of Archie formula, and puts forward different views on the application of the Archie formula in argillaceous sandstone and complex lithology of reservoir, low resistivity reservoir, which serves as a reference for the debate about relevant issues in recent years.