The invention provides a method for preparing stock solution dyed elastomeric polyester fiber. The method includes: preparing a colorant and an elastomer toughening agent into PET (polyester) masterbatch; then adding the PET masterbatch into a PET spinning stock solution for stock solution dyeing to obtain the dyed elastomeric polyester fiber directly by spinning. In the method, the colorant and the elastomer toughening agent are taken as the masterbatch and added to the spinning stock solution of the polyester fiber, and technical problems existing in dyeing properties and resilience broughtby the fact that traditional fiber needs to be dyed with dyes after forming are solved; the method is simple in operation and favorable for environmental friendliness, and the obtained polyester fiberis high in color fastness, bright in color, soft in handfeel and good in resilience.