The invention discloses a harmless pretreatment medicament and a water treatment method for fracturing flowback fluid. The medicament comprises a compounding gel breaker and a compounding flocculant which are separately prepared and stored before being used, wherein the compounding gel breaker is prepared from the following components in percentage by mass: 10-25% of potassium persulfate, 20-45% of calcium oxide and 30-60% of an enzyme gel breaker; and the compounding flocculant is prepared from the following components in percentage by mass: 10-25% of aluminium sulphate, 20-55% of iron sulfate heptahydrate, 15-20% of poly acrylamide and 15-35% of floridin. The water treatment method comprises the four steps of preparing a compounding gel breaker solution; preparing a compounding flocculant solution; calculating the dosage of the medicament, and sequentially adding the compounding gel breaker solution and the compounding flocculant solution; and enabling the medicament to react in waste water. The harmless pretreatment medicament disclosed by the invention has excellent treatment effects on the fracturing flowback fluid with large fluctuation of water quality.