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Manual sowing of Sinonovacula constricta seedling in tidal flat is a slow and labour-intensive operation, and damage of mechanical sowing limits the production of Sinonovacula constricta. The mechanical properties of 2-month-old Sinonovacula constricta seedling were tested and a pneumatic seeding device was proposed which mainly consisted of a seedling box, a grooved pulley, seedling-blowing pipe, and an air flow distribution pipe. Sinonovacula constricta seedling in the seedling box entered the seedlingblowing pipe via the grooved pulley, and then with the effect of air flow, they were blown and scattered onto the tidal flat. The kinematic model of seedling in the seedling-blowing pipe and air were established respectively. The contact forces of the collision process were obtained and the main factors influencing the movement trajectories of Sinonovacula constricta seedling were analyzed. Based on CFD-DEM method, single-factor and three-factor and three-level quadratic orthogonal central composite experiments were conducted respectively, with the inlet velocity of air flow, the distance from the ground, and the speed of the device selected as experimental factors, and the relative error of seeding density and the damage rate as performance indicators. The results showed that the factors affecting the relative error of seeding density, in order, were the speed of the device, the inlet velocity of air flow, and distance from the ground. The factors affecting the damage rate were the inlet velocity of air flow, the distance from the ground, and the speed of the device in order. The optimal combination of parameters was determined to be 30m/s of the inlet velocity, 490mm of the distance from the ground, and 0.17m/s of the speed. Based on the parameters obtained in simulation experiments, a prototype pneumatic seeder was fabricated and tested for its performance. The seeder worked satisfactorily at the relative error of seeding density of 7.02% and the damage rate of 3.92%. The research result can provide a technical support for the seeding devices of thin-shelled shellfish seedling on tidal flats.