The existing evaluation method of stratum thermal efficiency is on the basis of piston steam displacement model and ignores the influence of steam overlap.By studying the stress and seepage velocity of the steam in inverted nine-spot well patternthe overlapping degree evaluation method is obtainedconsidering different steam frontiers between the producer and the injector.The evaluation model for reservoir thermal efficiency is received on account of the study of steam overlapping degreeheat balance theory and the instantaneous heat balance expressionand solved by Laplace transformations and Stehfest numerical inverse method.The results show that in the early stages of steam injectionthe steam overlap degree is small and the steam chamber expands evenly on longitudinalso steam overlap has little influence on the thermal efficiency；in the middle and later stages of steam injectionwith the expansion of the steam chamber and the increase of steam overlap degreethe steam displaces along the topso steam overlap has severely impact on the thermal efficiency of the reservoir.