The invention discloses a method for determining reliability of a nickel electrode ceramic dielectric capacitor for space navigation. The grain size of a capacitor dielectric layer is accurately measured under the condition that the original grain state is not changed, and the capacitor reliability is calculated on the basis. The method comprises the following steps: performing section preparation by adopting a low-stress grinding technology so as to obtain a sample surface without stress residues; analyzing the sample by adopting an EBSD (Electron Back-Scattered Diffraction) technology, determining the type of ceramic dielectric grain phases, and collecting the grain distribution map on the sample surface so as to accurately measure the ceramic dielectric grain size; and finally, calculating the capacitor reliability by combining parameters such as the quantity of dielectric layers, the dielectric layer thickness, a working voltage coefficient and the like, thereby obtaining the capacitor reliability.