The invention discloses an online multi-band optical size detection method which is characterized by comprising the following steps: (1) setting a set of condensing device, wherein the condensing device comprises a plurality of independent condensing units distributed into a semicircle or circle, and each condensing unit comprises an LED (light emitting diode) light source and a condensing prism with an elliptical arc surface; (2) arranging two separate focuses on the elliptical arc surface of each condensing unit; (3) forming the LED light source of each condensing unit by using LEDs with the same band or different bands, wherein the bands of the LED light source cover the entire visible and invisible light regions; 4) arranging a detection object in a detected zone; and 5) arranging a set of magnifying glass below the detection object, amplifying the optical properties excited from the detection body, and analyzing the optical properties by using a computer system to obtain a detection result. The invention further discloses a system for implanting the method, which can be applied to the deviation correction and size measurement system of various plates.