The invention provides a method for treating waste acid into calcium hydrophosphate fertilizer, which comprises the following steps: mixing the waste acid with a temperature between 60 and 80 DEG C and powdered rock phosphate in a mass ratio of 100 percent sulphuric acid converted from the waste acid to the powdered rock phosphate being 0.65-0.75:1, and reacting for 4 to 6 hours in the condition that the concentration of liquid-phase SO3 is 30-50 g/liter; separating, filtering and washing reaction slurry to obtain filtering and washing mixed solution and filter residue containing ardealite, and drying the filter residue to obtain an ardealite product; performing neutralization treatment on the mixed solution, filtering and separating to obtain filter liquor and filter residue, and drying the filter residue to obtain fertilizer level calcium hydrophosphate; and discharging the filter liquor after the filter liquor reaches the standard or recycling the filter liquor. The method has simple process and low cost, recycles all the filter liquor, does not discharge any pollutant, simultaneously uses heat sources from industrial tail gas, has obvious energy-saving and consumption-reducing effects, treats the problem of the industrial waste acid, simultaneously obtains the fertilizer level calcium hydrophosphate, and has considerable economic and social benefits.