Abstract: It is found out that the viscosity of the remaining drilling fluid is higher than before the API loss test was taken. To explain this phenomenon, the paper presents a different opinion about the filtration mode used in the derivation of equation for calculation of static loss of drilling fluid, and derived a new equation based on a new filtration mode, which can explain the phenomenon. According to this new equation, twice of the result of 7.5 min test can replace that of the 30 min test in the API loss test.