Provided are a human PD-1 knockdown siRNA, a recombinant expression CAR-T vector, a preparation method thereof, and an application of the same. A PD-1 knockdown siRNA expression cassette and an siRNA expression product thereof can be applied to a CAR-T therapy of multiple myeloma (MM) for eliminating or alleviating a tumor immune escape mechanism, and in the suppression of an immune escape mechanism in a CAR-T therapy of a tumor, such as pancreatic cancer, brain glioma, and myeloma.