The invention discloses a method for quickly reinforcing an ultra-soft soil foundation by using a bagged sand cushion, which comprises the following steps of: (1) forming a quick reinforcing device on the ultra-soft soil foundation, namely (A) laying filling bags on the ultra-soft soil foundation in a reinforcing area, splicing the filling bags into a whole, filling fillers into the filling bags by using conveying equipment, tightening the filling bags after filling is finished and draining and compacting to form a bagged sand hard shell layer on the ultra-soft soil foundation, (B) inserting vertical plastic drainage plates into the surfaces of the filling bags on which the hard shell layers are formed, and (C) connecting a horizontal drainage pipeline with a vacuumizing system through a corrugated pipe to form the ultra-soft soil quick reinforcing device; (2) vacuumizing the quick reinforcing device; and (3) integrally reinforcing the foundation. The method has the advantages that: the process is simple, low in cost and short in construction period, differential settlement is small after construction, and the cost of laying a medium-coarse sand drainage cushion at a later stage is saved.