The invention provides a flexible package lithium ion battery and a processing method thereof. The flexible package lithium ion battery comprises a membrane housing, a lug, and an electric core and an electrolyte which are arranged in the membrane housing, wherein the membrane housing is provided with a sheet one-way valve, one end of the sheet one-way valve is inserted into the electric core and the other end of the sheet one-way valve is exposed outside the electric core, and the sheet one-way valve is used for exhausting gas inside the flexible package lithium ion battery. The processing method of the flexible package lithium ion battery comprises the following steps of: A, punching the housing without an air bag; B, coiling the electric core into the membrane housing; C, doubling over along an upper die cavity and a lower die cavity, carrying out tip seal and side seal, packaging the sheet one-way valve on the membrane housing; D, injecting electrolyte, standing until the electrolyte is absorbed completely and vacuumized; E, packaging the electrolyte-injecting side; and F, degassing. Through arranging the sheet one-way valve on the top, gas generated during the degassing can be immediately exhausted, the air bag and the air bag and the right side part of the air bag can be removed during punching the housing, thus raw materials and processing procedures are saved, and packaging effect and safety of the battery are improved.