The size at first maturity was one of the most important biological characteristics for fishery population. In order to know the size at first maturity of mantis shrimp (Oratosquilla oratoria), Huxley’s traditional model and polyphsic model were established for mantis shrimp collected from fishery independent surveys by trawl net in Bohai Bay from Apr. to Aug. 2018, and the size at first maturity was estimated. Our analyses showed that the polyphasic model was more suitable for fitting the length–weight relationship for mantis shrimp. According to Huxley’s traditional model, there were two independent spot clusters for the proportional residuals, which characterized two length–weight relationship groups. According to the polyphsic growth model, the sizes at first maturity were estimated as 10.99 cm, 11.01 cm and 10.85 cm for total, female and male mantis shrimp, respectively. These data suggest that a change in growth pattern could be related to a change in energy allocation related to the reproductive activities. When mantis shrimp, especially female, were in the period of maturity, more energy was allocated for sexual maturation, and perhaps decreased energy for somatic growth.