The work aims to deposit a dense and uniform thin copper layer of high conductivity on the surface of polyimide baseplate. Formaldehyde solution system taking copper sulfate as copper source was used to deposit copper layer on the surface of pretreated polyimide (PI) film. Surface roughness of the PI baseplate before and after pretreatment was analyzed with atomic force microscope (AFM) and contact angle meter (CAM). Surface morphology of the copper layer was observed with scanning electron microscope (SEM). Sheet resistance and shielding effectiveness of the as-prepared copper layer was tested with four probes tester and flange coaxial tester, respectively. The contact angle between PI baseplate and water reduced from 38° before pretreatment to 27° after pretreatment. The microstructure of electroless copper layer was compact and uniform. The sheet resistance decreased as the plating thickness increased, and the copper layer exhibited excellent electroconductibility. The maximum shielding effectiveness of copper plated PI baseplate could reach up to 55 dB in the scope of 100 kHz~12 GHz. Simple baseplate pretreatment and controlled copper plating time can contribute to deposition of quality copper layer on PI baseplate, and the shielding effectiveness meets the commercial standard.