We propose an ultrasensitive leaky surface acoustic wave (LSAW) immunosensor based on molybdenum disulfide @ cuprous oxide—gold (MoS2@Cu2O-Au) nanoparticles and subsequent gold staining for the detection of alpha-fetoprotein (AFP). MoS2@Cu2O-Au nanoparticles, with their large specific surface area and good biocompatibility, not only capture the secondary antibodies (Ab2) but also amplify the mass loading effect of the acoustic wave sensor in the detection of AFP. The immunosensor signals are further amplified upon injection of gold staining solution. The developed immunosensor achieved a low detection limit of 5.5 and 25.0 pg/mL with and without gold staining, respectively. The immunosensor demonstrated its efficiency for the quantitative detection of AFP in complex biological fluids, including human serum and saliva samples, with excellent selectivity and long-term stability, showing great potential for the quantification of AFP in clinical diagnosis.