Abstract: In order to determine the trawl depth quickly, improve the fishing efficiency and reduce the cost of fishery production, the paper proposes a method for calculating the optimal fishing depth of the specified fish target based on the sonar device metadata. Sonar device metadata structure is relatively complex and contains much redundant data. In the paper, the original data were simplified, and the information of seabed depth and target strength was calculated and extracted. The effective data range and noise data range were determined according to the type of target fishery resources. After filtering the noise data, the effective data was displayed in the form of statistical chart. The target fishery resources of each depth were counted. The relationship between depth and target fishery resources was constructed, and the optimal fishing depth was calculated and predicted by various methods. The results show that the optimal fishing depth of krill (Euphausia superba, target intensity −69.5–−40.8 dB) was 172.9–187 m in the survey area. According to the sonar data obtained in a certain sea area over a period of time, the optimal fishing depth of the target fishery resources in that sea area can be calculated quickly.