Abstract：Super-magnesium peridotite is the main host rock of boromagnesite ore in the Houxinyu boron deposit, Yingkou, Liaoning Province. The geological and geochemical characteristics of the rock are as follows: ① The rock occurs as concordant belts and lenses in magnesium-rich borate rocks of a boron-bearing rock sequence of the Paleoproterozoic volcano-sedimentary rock series, and is almost all made up of forsterite and has a blastic exhalative-sedimentary fabric. ② The major elements indicate that the rock is rich in magnesium and iron. ③ Inactive trace elements such as Zr, Hf and Ta are rich, which is similar to the case of typical ultramafic rocks; the lower contents of Cr and Ni suggest a non-primitive magma, and Nb*> 1 and the (La/Yb) N-(Yb) N diagram both indicate a mantle origin for the rock. ④ The total REE content ranges from 7.41×10-6 to 46.82×10-6, being similar to that of the upper mantle; the fractionation of LREE and HREE is not pronounced, with a pronounced negative Eu anomaly; and the chondrite-normalized REE patterns of the rock are “v”-shaped, being similar to those of dunite. ⑤ The values of εNd range from -7.51961 to 9.0654, which indicates the feature of mixing of the mantle and crust. From the above-mentioned geological and geochemical characteristics, it may be inferred that the rock is magnesium-rich rock left after relatively high degrees of partial melting of mantle peridotite contaminated by the crust.