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Tree-ring density has been suggested to be a useful proxy for revealing long-term late summer or growing season temperature variability on the Tibetan Plateau (TP). However, the current chronology length of tree-ring density records on the TP is obviously shorter than both tree-ring width and tree-ring isotope records. This phenomenon implies the possibility of extending the current tree-ring density record into earlier times and provides us with an opportunity to evaluate recent warming on a longer time scale. In this study, we present a 640-year tree-ring maximum latewood density (MXD) chronology of Balfour spruce back to 1,375 CE in the eastern TP. This tree-ring MXD chronology is longer than the previous ones on the TP. Growth-climate relationship analysis shows that the established MXD chronology correlates significantly and positively with August-September mean temperatures recorded at the nearest meteorological station over the common period of 1954-2014 (r = 0.66, effective degree of freedom [edf] = 62.7, p < 0.0001). Based on the growth-climate response relationship, the August-September mean temperature was reconstructed from 1,625 to 2,014, which is the reliable period of the MXD chronology, with the expressed population signal (EPS) greater than 0.85. The reconstructed August-September temperatures show an increasing trend since the late 19th century, and this reconstruction suggests that the last 10-, 30-, 50- and 100-year periods all appear to be the warmest over the past 390 years. Together with preciously published temperature reconstructions based on tree-ring MXD near our study area, the results of this study demonstrate that the southeastern TP has experienced post-industrial warming since the late 19th century.