The invention discloses a preparation method of a phosphomolybdate electrocatalyst for quantitative detection of Cr (VI) in water body. The preparation method comprises the following steps: dissolvingNa<2>MoO<4>*2H<2>O, FeCl<2>*4H<2>O and 1, 3-bis (4-pyridyl) propane (bpp) in water at room temperature, sequentially adding absolute ethyl alcohol and H3PO4, and regulating the pH value of an obtained reaction solution to 3.0 or so by using a NaOH solution; putting an obtained mixed solution into a high-pressure reaction kettle, heating for 5 days in a drying oven at 160 DEG C, slowly cooling toroom temperature, washing and filtering to obtain an orange red crystalline catalyst. The method is simple in equipment, low in cost and free of environmental pollution. The prepared material can effectively detect Cr (VI), the detection limit of Cr (VI) is as low as 0.009 ppm, and the catalyst has high stability and selectivity. Compared with the prior art, the method has the advantages of low catalyst consumption, greenness, high efficiency, low synthesis cost and sensitive response process.