The invention discloses a method for positioning and erecting suspension bridge main cable prefabricated parallel cable strands based on a scale cable strand. The method comprises the following stepsthat: the scale cable strand is installed in a scale cable strand saddle groove base, and a saddle-entering position marking line is marked; the prefabricated parallel cable strands are transversely moved into the prefabricated parallel cable strand saddle groove base, and cable strand discharging marking lines are aligned; the height difference between the midspan position scale strand and the prefabricated parallel strands is measured; according to the height difference, the adjustment amount of the prefabricated parallel cable strands is obtained through catenary numerical solution; an in-saddle positioning mark line is marked according to the adjustment amount by taking the in-saddle position mark line as a reference; and the prefabricated parallel cable strands are transversely movedinto a saddle according to the saddle-entering positioning mark line. High-precision positioning and saddle entering of the prefabricated parallel cable strands are achieved, repeated position adjusting operation after the prefabricated parallel cable strands enter the saddle is avoided, then traction erection and positioning saddle entering of the prefabricated parallel cable strands can be synchronously conducted, and erection construction efficiency is greatly improved.