The invention discloses a navigation satellite constellation availability assessment method based on the dynamic reliability estimation. The navigation satellite constellation availability assessment method comprises the steps of introducing a random failure factor, a degradation failure factor and a consumption failure factor of a satellite according to current operation data of an in-orbit constellation to establish a dynamic reliability estimation model and obtaining a time-varying curve of the single-star fault rate according to reliability estimation; then calculating out the constellation fault probability Pk,n (t) and the constellation inherent availability CVn,k value of an n kind of fault combination in the situation where k satellites in M satellites in the constellation are in fault; finally calculating out a time-varying relation of the constellation service availability of the navigation satellite system according to the Pk,n(t) and the CVn,k. The navigation satellite constellation availability assessment method can predict in-orbit reliability of the satellites in real time, can obtain high-accuracy and high-reliability satellite constellation availability and can solve the problems that a method only relying on a constellation configuration design to calculate out constellation availability is inaccurate and incomplete and cannot assess the constellation availability in real time.