The invention discloses a double-wall closed soil sampling method and device for soft soil. The method comprises the steps that A, double-layer sampling pipes are connected to soil sampler connectors and fixed through pins; B, a double-wall closed soil sampler is placed down into the bottom of a hole and pressed into soil for sampling; C, in the soil sampler pressing-in process, a lifting rod moves downwards along with the soil sampler; D, after the soil sampler is pressed to reach the depth, the soil sampler is pulled out upwards after the sampling pipes are closed; E, after the soil sampler is lifted to the ground, whether the sampling pipes deform or not is detected; F, soil samples are conveyed to a laboratory, closed and kept upright after being sampled out. According to the double-wall closed soil sampling method and device for soft soil, the inner and outer layer sampling pipes are connected with the sampler connectors through the pins, the inner layer sampling pipe is formed by splicing five identical rectangular steel plates along long edges in a contact mode, and a soil breaking piece is located at the lower end of each steel plate and connected with the inner layer sampling pipe through a hinge. The method is easy to implement, operation is easy and convenient, the structure is simple, stability and reliability are achieved, the soil sampling quality is high, and the sampling rate is improved.