According to Non-Darcy flow theory and potential superposition principle, this paper deduced pressure gradient distribution law at the normal production in square inverted nine-spot well pattern, and applied the result to the studies of effective injection-production system formation conditions and producing degree of water flooding reserves in square inverted nine-spot well pattern. Taking actual oilfield as an example, this paper studied the rule that producing degree of water flooding reserves changes with injection-production differential pressure variation, which can provide theory basis for adjustment well deployment in square inverted nine-spot well pattern.