The invention discloses a non-interventional fire hydrant water pressure detection method based on time-delay accumulation of multiple ultrasonic signals, comprising steps: S1. Using an ultrasonic transducer to generate multiple frequency-agile ultrasonic longitudinal wave sinusoidal excitation signals as input to detect ultrasonic signals; S2 . Acquire all reflected echo signals and transmitted echo signals corresponding to the input detection ultrasonic signal; S3. Accumulate the time delay interval of all acquired reflected echo signals and transmitted echo signals; S4. Accumulate all acquired reflected echoes The transmission distance difference between the signal and the transmitted echo signal; S5. Estimate the accumulated delay interval and transmission distance difference to obtain the ultrasonic wave velocity, and correct the obtained ultrasonic wave velocity; S6. Detect the water pressure of the fire hydrant according to the corrected ultrasonic wave velocity. The invention can realize the non-intervention, real-time, accurate, convenient, fast, efficient and cheap fire hydrant water pressure detection, and meets the practical application requirements of the fire hydrant water pressure detection.