The propose of this research is to study on the temporal-spatial variation of chlorophyll a concentrations in the Fei-Tsui Reservoir and the mechanism of how typhoons impact on it. The surface water temperature (ST), chlorophyll a (Chl a), phaeopigments a (Phae a) ratio (PhR= Phae a/(Phae a+Chl a)), total suspended matters (TSM) and soluble reactive phosphorus (SRP) have been measured in the Fei-Tsui Reservior from 2004 Feb to 2005 Apr, especially made high frequent sampling after typhoons. The result during the researching period told that the vertical distribution of Chl a concentrations was higher at the surface but lower at the bottom. The highest Chl a concentrations often located at 5m depth of water. The horizontal distribution of the Chl a concentrations was higher in the upstream but lower in the downstream. About the temporal distribution, the average of integrated Chl a concentrations above 10m (IChl a10m) had the highest value in autumn (5.2 mg L-1) and the lowest in winter (0.24 mg L-1). For its annual average value was 2.3 ± 1.2 mg L-1. The multiple regression analysis shows that the change of IChl a10m during research is related with ST and PhR (indicated the grazing of the zooplankton or the descending of the phytoplankton). The term TSM is considered with IChl a10m during typhoon season from Aug to Dec. The term SRP reveals related with IChl a10m only during cold season from Dec to next Mar. After typhoon or heavy rain, high TSM turbidity current was presented in the middle and bottom water (about 20~80m water depth) of the reservoir; the Chl a concentrations rose at the upper water body of the turbidity current, which the depth of subsurface chlorophyll maxima appeared deeper. The data analysis shows that TSM is iii positive related with SRP. That indicates the turbidity current can be a source of phosphate. It could be one reason of why the high Chl a concentrations presents during the typhoon season. The numerical model analysis shows that the strong breeze of typhoon can’t draw the material of deep water up because the water temperture stratification is so strong in summer. But the stratification is weaker in the late autumn that typhoon can make the mixing layer deeper efficaciously and causes the material of deep water is brought into upper water body. As these results point out, if the SRP or Chl a concentrations reveal high value in summer, the input should come from upstream. If the high TSM and SRP water induced by summer typhoon accumulates in the reservoir, it will mix with surface water causing by the typhoon in autumn or surge in winter and will affect the magnitude of the bloom in this autumn or next spring.