As key terrestrial ecosystems and natural resources in China, grasslands can provide provisioning, supporting, regulating, and cultural ecological services, which are regarded as highly multifunctional ecosystems. It's of great significance for grassland protection, restoration, and sustainable management to assess ecosystem service value (ESV) of grasslands. However, currently, there still lacks a systematic and accurate assessment framework in service values of grassland ecosystem. This study focuses on the northern grassland of China and taking the natural grassland in the Inner Mongolia as a typical case area, we propose a calculation framework for assessing grassland ESV index, comprising 19 different indicators for provisioning, supporting, regulating and cultural ESV. Based on the multi-source data such as ground sampling, field observation, remote sensing interpretation, model simulation, and statistics, we calculated the grassland ESV at the grid scale from 2000 to 2020 and then analyzed their spatial differentiation characteristics and dynamic evolution. The results showed that the mean annual grassland ESV in the Inner Mongolia during the last 20 years was 1764.06 billion yuan/a, equivalent to 2.05 times of GDP (8607.23 billion yuan/a) in the Inner Mongolia during the same period, showing a spatial distribution pattern of "high in the northeast and low in the southwest". The ESV provided by grassland per unit area was 23400 yuan/hm². Among them, 53.68% came from grassland regulating services with the highest mean annual value of 6676.44 billion yuan/a, mostly provided in Hulunbuir City, Xing'an League, and Xilingol League, etc. 35.13% came from grassland supporting services with a multi-year average of 6293.96 billion yuan/a. The multi-year average values of grassland provisioning and cultural services were 3796.27 billion yuan/a(6.24%) and 873.93 billion yuan/a (4.95%), respectively. From the perspective of specific indicators, the wind erosion prevention services in grassland had the highest value of 5858.86 billion yuan/a and occupied 33.21%. From the regional perspective, Xilingol League had the highest grassland ESV of 4701.81 billion yuan/a and occupied 26.65%. Over the past 20 years, grassland ESVs showed a significant increase trend of 439.45 billion yuan/a, among them the cultural services increasing mostly. This method and case study can provide a reference paradigm for assessment of grassland ecosystem values, contribute to establish a sound mechanism for realizing ecosystem product value, and promote the optimization of ecological compensation schemes such as grassland encroachment and occupation.