Aiming at the defect that most polarization-maintaining fiber input beam expander collimators used in cold atom experiments can not accurately adjust the polarization of the input laser beam, a compact polarization-adjustable laser beam expanding collimator with a full length of 135 mm and an effective diameter of 20 mm for the output circular spot is proposed. The polarization axes of the polarization prism and the wave plate in the beam expander collimator can be independently adjusted, and the polarization state of the input beam of the single-mode polarization maintaining fiber can be accurately adjusted and maintained. The developed laser beam expanding collimator has prepared cold atomic groups that meet the requirements of cold atom interference experiment in the cold atom experiment of three-dimensional magneto-optical trap. The atomic number of cold atomic groups was 5×10⁸, the temperature was about 10 μK, and the flight time signal of atomic fountain with the maximum throw-up height of 1.156 m was obtained.