The coleopteran parasitoid Dastarcus helophoroides Fairmaire (Coleoptera: Bothrideridae) is considered as a dominant natural predator for controlling cerambycid borer pests. In order to clarify the mating competency and the role of multiple mating behavior in the population reproduction of this insect, the sexual maturity, mating frequency, copulation duration and copulation interval of this insect were estimated, then the effect of multiple mating on adult reproductive output were also explored by established four mating treatments of mated 1 time, 5 times, 10 times and free mating under constant conditions in this paper. The results showed that the average sexual maturity period of this insect was 29.35 ± 0.34 d, and the mating behavior was mainly occurred from 18:00 pm to 2:00 am. During the observation period, the average mating frequency of the insect was 4.49 ± 0.24 days, with an average copulation duration of 9.27 ± 0.12 min and average copulation interval of 8.60 ± 0.26 d, which showed a significant shortening trend with increasing mating frequency. Furthermore, with increasing mating frequency, fecundity of this beetle increased from 227.45 ± 15.32 eggs/female for one mating to 1426.03 ± 113.71 eggs/female for free mating, and fertility increased from 68.93% ± 1.47% of one mating to 94.98% ± 0.90% of free mating. These results demonstrate that this beetle belongs to typical nocturnal, income breeding, and multiple mating insect species, and have weaker mating competency. But multiple mating significantly increases the reproductive fitness of this insect, supporting ejaculate replenishment hypothesis.