The present disclosure provides a method for manufacturing a gate stack structure and adjusting a gate work function for a PMOS device, comprising: growing an ultra-thin interface oxide layer or oxynitride layer on a semiconductor substrate by rapid thermal oxidation or chemical method after conventional LOCOS or STI dielectric isolation is completed; depositing high-K gate dielectric and performing rapid thermal annealing; depositing a composite metal gate; depositing a barrier metal layer; depositing a polysilicon film and a hard mask and then performing photolithography and etching the hard mask; removing photoresist and etching the polysilicon film, the barrier metal layer, the metal gate, the high-K gate dielectric, and the interface oxide layer in sequence to form a gate stack structure of polysilicon film/barrier metal layer/metal gate/high-K gate dielectric; forming spacers, source/drain implantation in a conventional manner and performing rapid thermal annealing, whereby while source/drain dopants are activated, adjusting of metal gate effective work function of the PMOS device is achieved.