The allocation and trade-off of plant productivity are hot topics in plant ecology. They could reflect the adaptability of plant to the environment, and also are the key to understand the response of plant to global climate change. To study the trade-off between of aboveground and belowground of plants is crucial to understand the survival strategies of vegetation and the sustainable developments of alpine grassland ecosystem on the Qinghai-Tibet Plateau, where are more sensitive to climate change. At present, the driving mechanisms of biomass allocation have been widely studied. Plant allocation patterns are generally characterized by plant root-shoot ratio and aboveground-belowground biomass ratio. However, these mainstream methods probably ignored the fact that belowground biomass of vegetation are accumulated over many years. Using aboveground and belowground net primary productivity data from 103 sample sites of alpine grassland on the Qinghai-Tibet Plateau, this study analyzed the trade-off between aboveground and belowground net primary productivity of two main grassland types, alpine meadow and alpine grassland. The influence mechanism of environmental factors on the trade-off of aboveground and belowground net primary productivity of two grassland types was explored by using relevant data such as climate factors and soil factors, combined with variance analysis, correlation analysis, dominance analysis and structural equation model. The results showed that:(1) The aboveground net primary productivity (ANPP), belowground net primary productivity (BNPP), and soil properties of the alpine meadow were significantly higher than that in alpine steppe (P<0.05); (2) In general, the trade-off of plant net primary productivity in alpine grassland tend to belowground (0.0199), and the trade-off to belowground in alpine steppe (0.0354) was higher than that in alpine meadow (0.0173); (3) The structural equation model showed that annual mean precipitation (AMP), soil bulk density (SBD), and soil available nitrogen (SAN) were the dominant factors affecting the trade-off in alpine meadow, and annual mean temperature (AMT) and AMP were the dominant factors in alpine steppe. The research showed that the productivity trade-off of alpine meadow was mainly affected by climate and soil factors, while alpine grassland was mainly regulated by climate factors. The findings of our study will provide a new insight and statistical approach for understanding the allocation between aboveground and belowground biomass of plants. In addition, our study will contribute to comprehending systematically the biomass allocation model, and predicting accurately the dynamic process of alpine grassland vegetation.