Soil conservation is one of the most important regulating functions of ecosystems. However, it has been negatively affected by climate change and human activities. Hainan Island harbors the most extensive primary tropical rainforest in China. Vast areas of these natural forests have been converted into commercial plantations in the last few decades. Increasingly, it is becoming apparent that nature reserves (NRs) play a key role in ecosystem services and are vital for prevention of soil erosion and upkeep of ecological balance. However, little is known about their effectiveness due to the lack of systematic planning and spatial data on their extent and boundaries. Evaluating the effectiveness of existing NRs in the preservation of soil conservation function is an urgent task. Here, using satellite images from 1988, 1998, and 2008, we evaluated the effectiveness of NRs in maintaining soil conservation function on the Hainan Island. Additionally, we analyzed the main driving forces involved in soil conservation that have changed during the last two decades. Our results showed that:(1) The average soil conversation capacity per unit area in NRs was 1951.59 t hm^-2 a^-1, which was 2.4 and 3.2 times larger than the average capacity registered 0-5 km and 5-10 km, respectively, away from the NRs and 2.9 times larger than the average capacity for the entire Island. Thus, NRs are critical for the maintenance of soil conservation function. (2) In the last two decades (1988-2008), soil conservation function showed a degradation trend inside and outside the NRs. Moreover, the degree of degradation outside NRs was significantly higher than inside NRs (P < 0.05), and it was significantly higher in the last 10 years of the studied period than in the first 10 years of the same period (P < 0.05). (3) From 1988 to 2008, economic development, population explosion, and expansion of cultivated land might have impaired soil conservation properties of the vegetation. Indicators such as gross regional product per area, population density, and area ratio of cultivated land were negatively correlated with soil conservation capacity (P < 0.05). Furthermore, in the first 10 years of the studied period, in addition to these factors, gross domestic product of the primary industry per area was also negatively correlated with soil conservation capacity (P < 0.05). Thus, tradeoffs should be made between protection of soil conservation function and human activities in order to attain coordinative development between eco-environmental protection and social economy.