The invention discloses a lithium ion battery self-discharge test method and a battery assembling method. By use of the feature that the lithium ion diffusion coefficient of an electrode material in a discharge platform area is quite small, the SOCs of batteries are tested through short-time pulse discharge tests, the self-discharge rates of the batteries are calculated, such that the batteries with consistent self-discharge performance are accurately and reliably sorted, battery packs are formed by effectively pairing the batteries, the overall service life of the battery packs is guaranteed, and the test method and the assembling method have a great practical significance for detecting the self-discharge capability of a power energy storage lithium ion battery system.