A laser-arc hybrid welding on-line monitoring method comprises the steps that S1, all devices are mounted and arranged; S2, testing samples are placed at designated spots; S3, multiple sets of hybrid welding parameters are randomly selected for machining, a temperature increase and decrease testing curve and a temperature distribution testing curve are obtained, and effective temperature intervals are determined; S4, if the difference values of the effective temperature intervals of the temperature increase and decrease testing curve and the temperature distribution testing curve are within the range a and the range b correspondingly, effective parameters are confirmed; S5, the multiple sets of effective parameters are averaged, and standard hybrid welding parameters are obtained; S6, the standard hybrid welding parameters are adopted for carrying out hybrid welding on an actual workpiece; S7, the step S3 is repeated to obtain a temperature increase and decrease actual curve and a temperature distribution actual curve, and effective temperature intervals are determined; and S8, if the difference values of the effective temperature intervals of the temperature increase and decrease actual curve and the temperature distribution actual curve are within the range a and the range b correspondingly, the welding requirement is met, and otherwise alarm information is fed back for data correction till the welding requirement is met.