Abstract: Robinia pseudoacacia and Armeniaca sibirica, commonly used as afforestation trees in semi-arid areas, were used as test materials, and 9 groups of treatments were set up, in which water-retaining agents (0 g plant⁻¹, 20 g·plant⁻¹, 30 g·plant⁻¹) were mixed with microbial inoculums (0 g plant⁻¹, 5 g·plant⁻¹, 10 g·plant⁻¹). The differences of plant functional indexes such as plant height, ground diameter and leaf area under different treatments were analyzed, which provided theoretical basis for the application of water-retaining agent and microbial inoculums in actual afforestation. The results showed 30 g·plant⁻¹ water-retaining agent (T7) concentration had the most significant effect on promoting the height of Robinia pseudoacacia, which was 2.69 times of the control group (T1). 20 g·plant⁻¹ water-retaining agent (T4) had the highest promoting effect on plant height of Armeniaca sibirica, which was 20.94% higher than control group. Under the treatment of 10 g·plant⁻¹ microbial agent (T3) and 5 g·plant⁻¹ microbial agent (T2), the ground diameter growth of Robinia pseudoacacia and Armeniaca sibirica was the largest, reaching 6.42 cmand 6.39 cm, respectively. Mixed application treatment had the best effect on the growth of leaf area, and Robinia pseudoacacia and Armeniaca sibirica reached the maximum value at T8 (30 g·plant⁻¹water retaining agent +5 g·plant⁻¹ microbial agent) and T5 (20 g·plant⁻¹ water retaining agent +5 g·plant⁻¹ microbial agent) respectively. Through K-value analysis, it was found that the effect of water-retaining agent on the growth of plant height and leaf area of Robinia pseudoacacia was more significant than that of microbial inoculants, and both of which showed the law of first decreasing and then increasing, but had little difference on the growth of ground diameter. For Armeniaca sibirica, the effect of microbial inoculants on the growth of plant height was more significant than that of water-retaining agent, and had little difference on the growth of ground diameter and leaf area.