Colloidal gold immunochromatographic strip is a simple and convenient method for detecting harmful substances, such as Morphine and CLB. For the purpose of reducing sterically hinder and enhancing the measurement sensitivity, 20nm colloidal gold was choosed. Therefor, in this paper, the microwave heating method was improved to synthesize the 20nm collidal gold, the addtion of reductantan and the heating time were explored. We use transmission electron microscope and UV-vis spectroscope to character the size of series particles. The result showed that when 1ml reduces was added and the heating time was more than 2min, the gold particles were close to 20nm and much more uniform. In conclusion, the improved microwave method was more suitable for preparation of small diameter gold particles.