本文使用LAMOST DR7星表与Gaia DR2星表进行交叉，从得到的星表中构建了一个高纯净度的，星等误差不超过0.002等,银纬绝对值大于20°的场星巨星星表和场星矮星星表。用这两个星表分巨星，矮星研究了BP-G 随BP-RP和[Fe/H]的变化关系，并据此利用Gaia的测光颜色来测量恒星的[Fe/H]。对巨星来讲，在BP-RP∈（0.7mag,1.4mag），[Fe/H]∈（-1.6 dex,0.3 dex）内的丰度测量误差约为0.25 dex，对于[Fe/H]<-1.6 dex的巨星误差约为0.5 dex，。对矮星来讲，在BP-RP∈（0.5mag,1.5mag），[Fe/H]∈（-1.6 dex,0.3 dex）内的丰度测误差约为0.25 dex，对于[Fe/H]<-1.6 dex的矮星误差约为0.5 dex。恒星越红，金属丰度越低，[Fe/H]误差越大

In this paper, we use LAMOST dr7 and Gaia DR2 to cross, and construct a high purity field star catalog of giant and drwaf with magnitude error less than 0.002, absolute value of galactic latitude greater than 20 °. We use these two catalogs to study the relationship among BP-G, BP-RP and [Fe/H] and then measured the [Fe/H] of stars by using the photometric color of Gaia according to this relationship. For giant stars, the error of metal abundance is about 0.25 dex in BP-RP ∈ (1.1mag,1.4mag), [Fe/H] ∈(-1.6 dex,0.3 dex) and about 0.5 dex for stars with [Fe/H] smaller than -1.6 dex. For dwarfs, the error of metal abundance is about 0.25 dex in BP-RP∈(0.5mag,1.5mag), [Fe/H]∈(-1.6 dex,0.3 dex) and about 0.5 dex for stars with [Fe/H] smaller than -1.6 dex. A star with larger BP-RP and lower metal abundance will get the larger [Fe/H] error.