Digital multi-beam forming has been widely used in ultrasound imaging system. It can effectively improve frame rate and image resolution. However, the mass of focusing delay data makes it impractical to directly implement dynamic focusing. A compression algorithm for four-beam focusing delay data is proposed to effectively reduce the storage of focusing delay data. Firstly, the calculation of four-beam focusing delay data is simplified to that of single-beam focusing delay data, which are then quantified, compressed and stored. When in dynamic receive focusing process, focusing delay data can be generated by decompressing the stored data for each channel. Take 8-channel 128-element linear array of transducers as an example, the design procedures and related mathematical derivation are described in detail and performance of the algorithm is discussed to verify the algorithm’s superiority.