A ring-width series was developed from Dahurian larch (Larix gmelinii) in the northeastern forest area of Inner Mongolia, China. By analyzing the relationships between tree-ring data and climate records, an August–September mean maximum temperature (T₈₉) series during 1845 and 2012 was reconstructed based on a simple linear regression equation. This reconstructed series explained 40.9% variance of the observed temperature from 1959 to 2012. The reconstructed T₈₉ series was consistent with the historical disaster events caused by extreme climate (e.g., flood, frost disaster, and cold damage). Besides, the temperature comparisons showed that the year in which the warm months (April–September) in northeast China began to warm up has latitude differences. It started with a gradual delay from north to south, starting 1980 in the south region, after 1950 AD in the central region and after 1940 in the north region. Our study can enrich high-resolution temperature series in Northeast China and help clarify the characteristic of recent warming in northeast China.