A synthetic process for gem grade diamond comprises the following steps: (1) placing an assembled synthetic block in a high pressure chamber of a cubic press; after pressurizing the synthetic block to a final pressure of 90-95 MPa at one time, maintaining the pressure; when the pressure rises to 40-50% of the final pressure, starting heating and enabling the heating power to rise to 4-5 kw within 10-15 s; after preserving the heat of the synthetic block for 3-5 min, increasing the heating power to 9-10 kw for heat preservation; (2) after preserving the heat of the synthetic block for 5-10 h, entering a continuous heat preserving and pressure maintaining stage after continuous temperature increasing, heat preserving, pressure increasing and pressure maintaining; (3) after continuous heat preservation, stopping heating and cooling the synthetic block to room temperature; releasing the pressure when the temperature is halved; finishing the process till the pressure is released to normal pressure. The synthetic process for the gem grade diamond is especially suitable for multi-crystal-seed synthetic processes, can reasonably supply enough carbon source according to the growing speeds of crystal seeds in the control process and can enable the crystal seeds to stably grow for a long time without being affected by spontaneous nucleation to synthesize large particle size and inclusion-free high-quality diamond single crystal; the crystal is colorless and transparent and the quality of the crystal is good; the synthetic process for the gem grade diamond can realize mass production and low production cost.