Abstract: The stability standards for plate load tests on soft foundation are studied based on the engineering practice. The differences of determing the characteristic values of bearing capacity and the ultimate values of bearing capacity among the slow method, the quasi-slow method and the quick method under different loading plates are analyzed. Through the analysis, the applicability of the extrapolation formula to the plate load tests on soft foundation is investigated. The extrapolation formula is improved. The extrapolation formula and the fitting method are proposed to describe the interrelation between the slow method and the quick method.