Abstract: To determine injection limits under different water injection conditions in an ultra-low permeability reservoir,based on the similarity theory model of one-dimensional waterflooding in consideration of the transient-state flow of a slightly compressible fluid,experimental parameters corresponding to field model were determined.According to laboratory results,similarity theory model was used to determine the relations between injection parameters under different water injection conditions.The results showed that:the waterflooding permeability boundary of ultra-low permeability reservoir increased linearly with the increase of well spacing under the conditions of given injection rate,and the waterflooding permeability boundary of ultra-low permeable reservoir increased linearly with the increase of injection rate under given well spacing.