In order to improve the comprehensive performance of portable media player video decoding, this study is optimal design of MPEG-4 video decoding system Based on embedded NiosII soft-core. To put forward the design scheme of software and hardware co-design in SOPC. By studying hardware IP cores optimized design and implementation of the two-dimensional inverse discrete cosine transform, motion compensation, color space conversion, a video decoding system in hardware and software co-design was build based on NiosII soft-core. As the core for model EP2C35F672C8 of Altera FPGA chip in SOPC system test，and the results show that the bit rate of the test stream is 1593.90 kbps, the frame rate can be achieved 35.20 frames per second at the operating frequency only 100MHz. It is good to achieve a real-time decoding of MPEG-4. The SOPC software and hardware co-design realize the player with the low power consumption and high performance.