Abstract: This article presents a study in order to solve the problem that the phase estimation cannot complete the application for weak GPS signal. Phase doubled processing and decision directed feedback were both used to eliminate the influence of the reverse of navigation data. A new maximum likelihood (ML) phase estimator based on incoherent integration with decision directed feedback was proposed. The expressions of SNR were derived in theory. RMSE was acquired by numerical integration of the conditional probability density function (CPDF). The new method could reduce RMSE of ML phase estimator and improve the capability of phase estimation for weak GPS signal. The effectiveness of theory analysis and the proposed algorithm were proved by simulation. The practicability of the algorithm is indicated by the result of the implementation based on field-programmable gate array.