The utility model provides an in-plane water flow rate testing instrument for geosynthetics. The in-plane water flow rate testing instrument for the geosynthetics comprises a high water tank, a low water tank and a pressurizing cavity body; the pressurizing cavity consists of a bottom plate and a top plate; a plurality of fixing screws are arranged about four sides of the bottom plate; the top plate passes through the fixing screw and are limited through screwing nuts which sleeve the fixing screws; an elastic pressurizing rubber film is arranged below the top plate; a pressurizing cavity is formed between the top plate and the elastic pressurizing rubber film; a sample positioning space is formed between the elastic pressurizing rubber film and the bottom plate; a pressurizing hole and a pressure release hole are formed in the top plate; the pressurizing hole is connected with a pressurizing pump through a pressurizing pipeline; an adjustable height bottom water inlet is formed in the part in which the high water tank is connected with the pressurizing cavity body; a first sealing gasket is arranged at the bottom water inlet; an adjustable height bottom water outlet is formed in the part in which the low water tank is connected with the pressurizing cavity body; a second sealing gasket is arranged at the bottom water outlet. The instrument is high in generality, simple and convenient to operate, and high in testing precision.