A novel label free electrochemiluminescence (ECL) immunosensor based on the ECL of peroxydisulfate solution for detection of α-1-fetoprotein (AFP) has been developed. For this proposed immunosensor, L-cysteine was firstly electrodeposited on the gold electrode surface, which promoted the electron transfer and largely enhanced the ECL of peroxydisulfate solution. Subsequently, gold nanoparticles (nano-Au) were assembled onto the L-cysteine film modified electrode to improve the absorption capacity of antibody and further amplify the ECL signal. Then, antibody was immobilized onto the electrode through nano-Au. At last bovine serum albumin (BSA) was employed to block the nonspecific binding sites. As a result, a novel ECL immunosensor was firstly obtained by applying the ECL of peroxydisulfate solution without conventional luminescent reagents. The AFP was determined in the range of 0.01-100 ng mL(-1), with a low detection limit of 3.3 pg mL(-1) (S/N=3). The proposed ECL immunosensor provides a rapid, simple, and sensitive immunoassay protocol for protein detection, which might hold a promise for clinical application. Moreover, this work would open up a new field in the application of peroxydisulfate solution ECL for highly sensitive bioassays.