An optical detecting technique to identify bio-aerosol particles is proposed in this paper by normalized fluorescence value which correlates to its size and intrinsic fluorescence. With the bio-aerosol detecting system developed, we test and analyze three types of aerosols, while each of them contains fluorescent microspheres of a certain size. The result indicates that different fluorescent microspheres containing the same fluorescent substances have the same normalized fluorescence voltage to unit particle size in diameter. The normalized fluorescence value of other species aerosols is tested for comparing. The research results can be applied to identification of bio-aerosols preliminarily.