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In this study, all unpublished time series photometric data of BM UMa ($q\sim$ 2.0, P = 0.2712\,d) from available archives were re-investigated togetherwith new data taken from the TNT-2.4m of the Thai National Observatory (TNO).Based on period analysis, there is a short-term variation superimposed on thelong-term period decrease. The trend of period change can be fitted with adownward parabolic curve indicating a period decrease at a rate of$\mathrm{d}P/\mathrm{d}t = -3.36(\pm 0.02)\times10^{-8}$ d $\textrm{yr}^{-1}$.This long-term period decrease can be explained by mass transfer from the moremassive component ($M_2 \sim 0.79 M_{\odot}$) to the less massive one ($M_1\sim 0.39 M_{\odot}$), combination with AML. For photometric study, we foundthat the binary consists of K0\,V stars and at the middle shallow contact phasewith evolution of fill-out factor from 8.8\,\% (in 2007) to 23.2\,\% (in 2020).Those results suggest that the binary is at pre-transition stage of evolutionfrom W-type to A-type, agreeing to the results of statistical study of W-typecontact binaries. The mass of $M_2$ will be decreased close to or below $M_1$and the mass ratio will be decreased ($q < 1.0$). By this way, the binary willevolve into A-type as a deeper normal over-contact system with period increase.Finally the binary will end as a merger or a rapid-rotating single star whenthe mass ratio meet the critical value ($q < 0.094$), as well as produce a rednova.

BM UMa: a middle shallow contact binary at pre-transition stage of evolution from W-type to A-type