This invention relates to a method for immunoassays based on infrared reflection absorption spectroscopy that utilizes proteins such as antigen and antibody that have specific absorptions in infrared absorption spectrum under 1550 cm−1. A chip is made from infrared reflective substrates such as a gold or silver plate, whereon the antibodies or antigens are immobilized by the specific bonding action between metal surfaces of the chip and proteins, and the protein signals are then detected by infrared reflection-absorption and infrared microscopy. Since infrared radiation is capable of interacting with any organic molecules, there is no need to label the samples with fluorescent reagents or nucleic irradiation reagent. This invention thus provides a label-free process that can avoid the inconvenience of labeling common biochemical material for detection in the prior art, thus achieving the purpose of higher speed of detection.