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A 12-month consecutive investigation was made at Jingha village in southern Yunnan of southwest China from April 2016 to March 2017. A total of 2053 Indochinese forest rats (<i>Rattus andamanensis</i> Blyth, 1860) were captured and examined, which account for 84.69% (2053/2424) of all the animal hosts (rodents and other small mammals) at the investigation site. And 39.82% (13,531/33,980) of gamasid mites were identified from the body surface of <i>R. andamanensis</i> and they belong to 41 species, 10 genera, 3 subfamilies and 2 families. Of the 41 species of gamasid mites identified from <i>R. andamanensis</i>, <i>L</i><i>aelaps nuttalli</i> Hirst, 1915 and <i>L</i><i>aelaps echidninus</i> Berlese, 1887 were the most dominant with 70.63% and 20.67% of constituent ratios respectively. In monthly fluctuations of all the gamasid mites on <i>R. andamanensis</i>, the constituent ratio (<i>C_r</i>) and overall infestation mean abundance (<i>MA</i>) of the mites in 12 months showed two obvious peaks in January (winter season) and June (summer season). However, the two dominant mite species, <i>L. nuttalli</i> and <i>L. echidninus</i>, showed different patterns of seasonal fluctuations. <i>Laelaps nuttalli</i> occurred throughout the year, and its <i>C_r</i> and <i>MA</i> showed two prominent peaks in winter season (December and January) and summer season (June), which belongs to the summer-winter type of seasonal fluctuation. <i>Laelaps echidninus</i> also occurred on <i>R. andamanensis</i> throughout the year, but its <i>C_r</i> and <i>MA</i> showed only one peak in winter season (December and January), which belongs to the winter type of seasonal fluctuation. A negative correlation existed between two climatic factors (temperature and rainfall) and the infestations (<i>C_r</i>, prevalence <i>P_M</i> and <i>MA</i>) of two dominant mite species (<i>L. nuttalli</i> and <i>L. echidninus</i>) on <i>R. andamanensis</i> (<i>p</i> < 0.05). Temperature and rainfall are considered to be two key factors that influence the seasonal fluctuations of the mites on the studied rat species.