A transparent simulating device was designed for investigating resident time of pyrolysis product of biomass in laminar entrained flow reactor. Resident time for char replacing pyrolysis product was measured using PIV (particle image velocimetry) at different gas flow rates and collection distances. Main gas flow rate, which measured at 1.0, 1.5, 2.0 and 2.5m3/h respectively, was determined by the similarity criteria that Reynolds number should be equal both in cold and hot flow field. Collection distances were 200, 250, 300 and 350mm. The results showed that the ratio of actual resident time of char and theoretical resident time of gas was proportional to Reynolds number, which meant the ratio of time and Reynolds number had one order functional relationship. Functional relation was different for different collection distances. It can therefore be concluded that when flow field is laminar, resident time of char can be calculated by functional relationship between ratio of time and Reynolds number. 