An ultrasonic phased focusing time delay system was established by using the multi-stage half-band filters to improve the accuracy of ultrasonic phased array instruments. The principle of the focusing time delay and the realization of the corresponding time delay algorithm were investigated. Firstly, the interpolation filter with a half-band one was used as an interpolation filter based on interpolation method. Then, the structure of the 8 × interpolation was improved. With a synthesis technique of a multi-stage half-band interpolation filter, the synthesized filter was divided into 8 sub-filters to work at the same time. In this way, both interpolation and polyphase decomposition could be simultaneously processed. Finally, the feasibility of this delay algorithm was verified by using the simulation analysis and the implementation of Field Programming Gate array(FPGA). The experimental results show that the time delay accuracy of the system is 1.25 ns at a sampling rate of 100 MHz. Comparing with the ordinary Finite Impulse Response(FIR) interpolation filter, the amount of computation with proposed interpolation filter can be reduced by 21.4% under the same condition. The program is characterized by lower computation amount, fast computing speed, excellent resolution and cost efficiency and can improve the overall performance for phased array instruments.