The invention discloses an isometric process control method utilizing PVT (pressure-volume-temperature) relation of polymers. An injection process includes a first step: a PVT curve of an injection molding material is obtained according to the category and the characteristics of the material, an A-B-C-D-E-F process route is obtained from the curve, and an A-B-C section is an injection molding stage; a second step: constant-pressure packing is realized under the maximum packing pressure for a period of time, then isometric packing is carried out, and a C-D section is an isobaric compression stage; a third step: a D-E section is an equal-ratio volume packing stage, a material in a cavity cannot be changed, and cannot backflow or be increased along with reduction of the temperature of the cavity, and packing pressure of melt is obtained by means of calculation by the aid of a Tait equation and physical parameters provided by a PVT database; and a final step: an E-F section is a constant-pressure cooling stage. The constant-pressure packing is realized before the equal-ratio volume packing is realized, the melt is in a flowing state during the constant-pressure packing, accordingly excessively packing alignment is avoided, warpage is low, and quality repeat precision is high.