Abstract: A single hydrophone is used for measuring noise radiated from underwater vessel as a traditional method. In order to measure low noise vessel, the method of using vertical array is developing in recent years. The measurement results of the two methods are consistent in free sound field, are different in waveguide. The purpose of this paper is to find out measurement error and its correction, and make the results of the two methods consistent. After defining parameters of a typical waveguide, the transmission functions from sound source to each hydrophone of a vertical array are calculated by wavenumber integration, and then the transmission function of focus beam of vertical array is deduced,.finally the measurement errors of the two ways are found. Numerical computation reveals that in the sea of 70 m depth, the measured sound level of a sine signal in different depth varies up 15 dB for single hydrophone measurement, up 7 dB for vertical array measurement. The mean of measurement error of wide band sound level trends to 3 dB for single hydrophone and to 0 dB for vertical array measurement. Additionally the paper gives the measurement result of sound level of a sine signal on an ocean experiment, which is consistent with numerical computation. The vertical array obtains obvious space gain in the ocean experiment. It is concluded that the measurement precision of vertical array is higher than that of single hydrophone, the measured wide band sound level by single hydrophone needs correction that is realizable, while the correction of the sine signal level by single hydrophone is very difficult.