The invention discloses an experimental device and method for modifying a thermal stress fracturing considered natural fracture developed geothermal reservoir of an enhanced geothermal system. The experimental device comprises a geothermal reservoir modification chamber, a prefabricated rock, a silica gel heating piece, a constant temperature liquid supply groove, a high pressure plunger pump, a first pressure intensity sensor, a first temperature sensor and a horizontal two-way crustal stress simulation device, wherein a closed cavity is formed by the geothermal reservoir modification chamber; one side of the geothermal reservoir modification chamber is provided with an inlet communicated with the cavity, the prefabricated rock is placed into the cavity, a prefabricated well shaft is arranged in the prefabricated rock, the prefabricated well shaft comprises an iron pipe and a hard plastic pipe, the iron pipe is communicated with the inlet, the hard plastic pipe is sleeved with the iron pipe, the hard plastic pipe and the iron pipe are fixed, the hard plastic pipe comprises an exposed section protruding out of the iron pipe, a plurality of naked eye holes are formed in the exposed section, and the naked eye holes are connected with a perforation pipe.