The invention discloses a gap pretreatment construction method for a foundation pit support structure. The gap pretreatment construction method for the foundation pit support structure comprises the steps that (1) a groove is excavated downwards along the foundation pit support structure in a first layer of an earthwork, and the leakage situation of a gap of the foundation pit support structure is observed; (2) pretreatment is conducted in a pertinence mode according to the leakage situation of the gap of the foundation pit support structure and the leakage gap is blocked off; (3) large area excavation for the first layer of the earthwork is conducted from the groove to the middle of the first layer of the earthwork after the leakage gap is blocked off; and (4) after excavation for the first layer of the earthwork is completed, a groove of the next layer of the earthwork is excavated downwards along the foundation pit support structure, and the steps from (1) to (4) are repeated until construction for a foundation pit is completed. According to the gap pretreatment construction method for the foundation pit support structure, gaps of the foundation pit support structure are timely discovered by utilization of the time-space effect principle for the foundation pit before large area excavation for the foundation pit; and pretreatment for the gaps can be timely conducted through foundation pit inside and outside treatment measures to reduce the problem of water and sand leakage in the gaps, lower engineering risks and save the engineering cost.