The invention discloses a preparation method for a graphite phase carbon nitride-doped bismuth molybdate binary photocatalyst with good performance. The method includes the following steps: step 1, weighing urea and melamine, adding the weighed urea and melamine into deionized water, after the urea and melamine are dissolved, putting the solution in a 100 ml reaction kettle, and performing a reaction at 180 DEG C for 24 h to obtain an A; step 2, cooling the A to room temperature, performing centrifugation, performing drying at 70 DEG C for 10 h, and performing two-stage calcination on the dried A at 520 DEG C for 4 h to obtain g-C3N4; step 3, dissolving bismuth nitrate pentahydrate in dilute nitric acid, adding ammonium molybdate, and performing heating and stirring at 40 DEG C to obtain asolution B; step 4, adding citric acid and the g-C3N4 into the solution B to adjust the pH to 6, adding 1.75 ml of ethylene glycol, performing magnetic stirring at 80 DEG C, transferring the mixed solution to a 100 ml reaction kettle, and performing a reaction at 150 DEG C for 8 h; and step 5, performing cooling to room temperature, performing suction filtration and drying, allowing the dried material to stand for overnight, performing calcination at 400 DEG C for 4 h to obtain the g-C3N4 / BMO. According to the method, doping is performed by using the modified bismuth molybdate, so that the recombination ratio of photogenerated electrons and holes affecting an interface reaction is reduced to improve the photocatalytic efficiency of the composite material.