The invention is applicable to the field of medical instruments and discloses a plasma extracting device. The plasma extracting device comprises a cavity where negative pressure can be formed, one endof the cavity is a whole blood extracting channel and a plasma output channel which are mutually insulated, the cavity is communicated with the whole blood extracting channel and the plasma output channel respectively through a first one-way valve and a second one-way valve, the tail end of the whole blood extracting channel is a blood collecting end, the tail end of the plasma output channel isa plasma output end, the plasma output channel is internally provided with a filter capable of filtering whole blood into plasma. When negative pressure is formed in the cavity, the whole blood is collected into the cavity through the blood collecting end; when pressure is applied to the cavity, the whole blood in the cavity is filtered into plasma and then discharged through the plasma output end. The plasma extracting device has the advantages that the whole blood can be collected at one time and the plasma can be output at one time, that a plasma sample can be obtained without other blood cell separation apparatus, that the operation is convenient, that the plasma can be extracted accurately and quantitatively, that the cost is low, and the device is safe and reliable.