A method for compositing trialkoxysilanesby adopting a multistage fluidized bed relates to a composite method of trialkoxysilanesby. The composite method solves the problems that in the existing composite technology of trialkoxysilanesby, catalytic agent is difficult to be recycled, reactive efficiency is low, products are difficult to be separated, silica powder and copper-based catalytic agent have poor contact, copper silicon contacts have slow productivity and low reactive activity, the catalytic agent loses effectiveness easily after meeting water, and the products are easily hydrolyzed after meeting water. The composite method of trialkoxysilanesby comprises the following steps: 1. preparing copper silicon contacts containing different cuprous chloride mass percents; 2. charging copper silicon contacts obtained in the step one to a reactor of the multistage fluidized bed in stages; and 3. introducing alcohol gas dried by ethyl orthosilicate into the reactor of the multistage fluidized bed to obtain trialkoxysilanesby after reaction and composition. By adopting the method for compositing trialkoxysilanes by adopting the multistage fluidized bed, the selectivity of trialkoxysilanesby reaches 95% and reactive transformation ratio reaches 95%.