The present invention provides an online measuring method of particle (such as bubbles, droplets and solid particles) velocity in multiphase reactor. The method based on an online multiphase measuring instrument includes the following steps: (1) the online multiphase measuring instrument is placed into the multiphase reactor, and then a particle image produced by two or more exposures are obtained; (2) the actual size of individual pixel in the particle image is determined; (3) valid particles are determined in the depth of field; (4) then the centroid coordinates are conversed to the actual length of the coordinates (xt,i, yt,i) and (xt+Δt,i, yt+Δt,i) using the actual size of individual pixel. Thus, the instantaneous velocity of particles can be calculated by V i = ( x t + Δ ⁢ ⁢ t , i - x t , i ) 2 + ( y t + Δ ⁢ ⁢ t , i - y t , i ) 2 Δ ⁢ ⁢ t . The method can realize real-time measurement of the velocity distribution of bubbles, droplets or solid particles in a multiphase reactor, and the measurement accuracy is high.