The invention provides an oil-gas well sand prevention method and device based on a geomechanical model. The method includes the steps that a calculation model of flowing bottomhole pressure of an oil-gas well is established according to a well surrounding rock stress analysis model of the oil-gas well and a rock failure criterion; geomechanical parameters are calculated according to logging information, wherein the geomechanical parameters comprise maximum horizontal principal stress, minimum horizontal principal stress, tensile strength of well wall rock and reservoir pore pressure; the flowing bottomhole pressure is calculated according to the geo-mechanical parameters and the calculation model of the flowing bottomhole pressure; the critical production pressure difference of the oil-gas well is determined according to the reservoir pore pressure and the flowing bottomhole pressure; the relation between the reservoir pore pressure and the critical production pressure difference in the production process of the oil-gas well is determined according to the logging information, and the actual production pressure difference of the oil-gas well is adjusted according to the relation. The critical production pressure difference of output sand of the oil-gas well is accurately predicted, and therefore effective sand prevention is carried out.