The invention discloses a sampling method for evaluation of an oil-gas-water multiphase fluid antiscale agent, which is characterized by sampling with pressure by using a pressure container, filling the antiscale agent; and according to the change of the field process flow parameters, adjusting the pressure in a sampler by opening and closing two valves of the sampler. By sampling and filling thechemical with pressure, the sampling method prevents CO2 in the water sample from releasing and causing crystal precipitation of CaCO3 and the like, so that the experimental data are more real and valid and have directive significance in the field antiscale.