Abstract: Leaf area index (LAI) objectively describes the number of leaves, is a key parameter controlling many biological and physical processes associated with vegetation on the earth's surface, such as photosynthesis, carbon and nutrient cycle, radiative balance, evapotranspiration, and rainfall interception. Thus, LAI is widely used in models of ecology, hydrology, biogeochemistry and climate. Accurately measuring LAI is one of the hotspots in the study of earth science. Long time series LAI is an important input parameter of daily forest carbon and water cycle models, and its accuracy directly influences the result of the model simulation. At present, site measurements of Long time series LAI data expend a large number of repeated manual operations, and produce a great error in the operation. An instrument, with which LAI can be indirectly measured in the field for a long time, was designed in this study. This instrument consisted of solar power system, single-chip microcomputer control system and time control system. The single-chip microcomputer control system was mainly made up of Arduino UNO microcontroller, clock module, storage module, display monitor module steering engine, light intensity sensor, temperature sensor and humidity sensor. In order to prevent the extra light intensity of a single direction that affected the accuracy of measurement, the measuring time was set at 7:00-8:00 and 16:00-17:00 by the time-control system. The solar batteries provided durable electric energy for the instrument. The vertical and horizontal rotary servos were controlled by microcontroller which drove the light sensor to record the light intensity from different solar zenith angles. It can record the light intensity from the zenith angles of 0°, 12°, 24°, 36°, 48°, 60°. Two instruments were used at the same time to measure LAI. One instrument was placed under the forest canopy in a plot to measure light intensity through the canopy, another instrument was placed in some open place, such as forest station, to measure light intensity equivalent to that above canopy. The light transmittance through the canopy was calculated by the ratio of the measured data of the two instruments respectively at different angles. LAI calculation was based on radiation transmission through the canopy. The commercial optical instrument LAI-2200 was used to test and verify the measurement precision of this instrument. LAI was measured on 24 plots, which were set in the experimental forest of Northeast Forestry University. The size was 20×30 m2. The minimum value of LAI was 1.625, the maximum was 6.323 and an average was 4.21 by using this instrument. The minimum value of LAI was 1.97, the maximum was 6.04 and an average was 3.99 by using LAI-2200. The research showed that the result of Long time series LAI measuring instrument was highly consistent with commercial optical instrument LAI-2200, with R2 of 0.927, RMSE of 0.36. The cost of the instrument was low, and with such instrument, network measurement would be possible among different forests. The stable operation of the instrument in high humidity environment of the forest was due to its waterproof design. When LAI was measured, the instrument collected data of air temperature and humidity to provide accurate environmental data for the study of long time series LAI changes. All collected data were recorded in the SD card, which was convenient for data application.