The invention relates to a treatment system of residual waste liquid in a membrane module and a control method of the treatment system. The treatment system is assembled on the original waste liquid treatment system and comprises a compressed gas inlet mechanism communicated with a first liquid inlet of the membrane module in the original waste liquid treatment system, and a waste liquid receiverwith a second liquid inlet, wherein the second liquid inlet is communicated with at least one first liquid outlet of the membrane module; and the compressed gas inlet mechanism is used for supplying compressed gas capable of providing an acting force of promoting the residual waste liquid to be discharged from the first liquid outlet to the second liquid inlet into the membrane module. The compressed gas inlet mechanism supplies the compressed gas into the membrane module to discharge the residual waste liquid from the membrane module to the waste liquid receiver; emptying of the residual waste liquid in the membrane module can be ensured; when the compressed gas is supplied into the membrane module for enough time, a membrane element in the membrane module can also be in a semi-dry state;the possibility of leakage of the residual waste liquid is not required to be considered any longer when the membrane element is dismounted from a membrane housing; and a dismounting speed of the membrane element is increased.