The invention relates to a multichannel micro-reactor system and method for preparing low-molecular-weight polystyrene. The system comprises a storage trough for polystyrene, a storage trough for a methylbenzene solution of an initiator, a micro-mixer separately connected with the storage troughs, inlet distribution channels successively connected with the output of the micro-mixer, a parallel reaction channel with a heat exchanging unit and an outlet collection channel, wherein reactants firstly pass through the micro-mixer and then enter the inlet distribution channels, the reactants are distributed in the micro-channel by virtue of a one-step distribution mode or a level-by-level distribution mode, the reaction temperature in the reaction channel is controlled by virtue of the heat exchanging unit, and the polystyrene reaction solution from the parallel channel is collected at the outlet collection channel. By adopting the multichannel micro-reactor system and method, the reaction time is shortened to 5 to 30 minutes, the monomer conversion rate is 90 percent or above, the molecular weight is 1000 to 10000, the distribution of the molecular weight is 1.1 to 2.5, and the yield can reach 10 kg/day.