Abstract: In order to clarify the optimal planting density of soybean variety Liaodou No.21 in Luoping County, Liaodou No.21 was taken as the experimental material, and five density treatments were set up by using the randomized block design, including Treatment A (20 plants·m⁻²), Treatment B (25 plants·m⁻²), Treatment C (30 plants·m⁻²), Treatment D (35 plants·m⁻²), and Treatment E (40 plants·m⁻²). Then, the effects of different planting densities on the growth dynamics, agronomic traits and yield of Liaodou No.21 were studied. The results showed that the planting density had a great influence on the yield of Liaodou No.21. When the planting density was 30 plants·m⁻², the total yield reached its maximum value of 2933.6 kg·hm⁻². The lower planting density resulted in the insufficient number of soybean populations, while the planting density was too high, it was difficult for the individual plants to obtain sufficient nutrients, energy, and water for their growth. Therefore, an appropriate planting density was crucial for improving the yield of Liaodou No.21.