Abstract: The investigation indicates that amphioxus (Branchiostoma belcheri) is mainly distributed within the area with 111°10′～111°17′longitude, 21°19′～21°27′ latitude, which covers 195.5km² in Maoming Dafangji sea areas. The average resource density is 51～86 ind/m² the highest is 815 ind/m². The area with a high density is centered in the west and northwest of the Xiaofangji Island. The body length distribution of amphioxus has an obvious seasonal fluctuation and the main recruit seasons are autumn and winter. The gonad maturity and sex ratio of population significantly change with seasons and growth stages and with a highest proportion of sex mature 65% in spring. Furthermore, along with the increase of the body length,the gonad maturity is increased gradually. Gonad maturity percentage is a function of binary cubic curve to the body length. Within the scope of 35～60mm of body length, the sex ratio of population approaches 1; if the body length is longer than 60mm the individuals of female are obviously less than those of male; the body length-weight relation equation of amphioxus is W=0.0012L^3.32. As the body length is longer than 50mm, the reducing coefficient of amphioxus population is 0.55/cm.