To improve the efficiency of microalgae photobioreactor to reduce cultivation costs and carry out large-scale cultivation, an online measurement method was proposed to measure the radiative properties of microalgae at whole growth phase. Using a custom flate-plate photobioreactor, an online measurement scheme for the radiative properties of microalgae was presented and the limitation of traditional measurement method was studied. An improved scheme was proposed based on the characteristics of radiative properties of microalgae. Chlorella vulgaris was taken as an example and its radiative properties at growth phase was experimentally studied. The results show that the proposed online measurement scheme can be effectively applied to measure the time-dependent radiative properties of microalgae. The improved scheme of spectral absorption and extinction coefficient measurement are more accurately applied to measure the radiative properties of microalgae at different growth phase with varied cell concentration. The spectral absorption and extinction corss-sections of Chlorella vulgaris vary significantly with growth time in the visible spectral range. This work provides a convenient and feasible experimental scheme for the measurement of radiative properties of microalgae at different growth period.