Disclosed is a chemical metrology discrimination method for plastic type which comprises: employing optical spectrum information of plastic infrared spectrum characteristic area, selecting characteristic area infrared spectrum as input variable of competitive back propagation artificial nerve network with nonlinear iterative least square variable selecting method; the competitive back propagation artificial nerve network comprises front end back propagation nerve network and back end competitive function layer, taking unit matrix as network training target, wherein the unit matrix having the amount of known plastic resin sample as rank, having training on network connection weight value with known type plastic sample input matrix and analog noise input matrix and establishing network connection; input variable being converted by back propagation network layer and competitive function layer and the most significant neuron being shown as result, therefore plastic type discrimination system being established. The inventive method has the advantages of being rapid and accurate.