A memory device and a forming method thereof are provided. The memory device includes: a semiconductor substrate, wherein multiple active regions are formed in the semiconductor substrate, and the multiple active regions are separated by multiple first trenches extending along a first direction and multiple second trenches extending along a second direction; a third trench, extending along the first direction and located in the semiconductor substrate at the bottom of the first trench; a bit line doped region, located in the semiconductor substrate on two sides of the third trench; a gate dielectric layer, located on a sidewall surface of the first trench and a sidewall surface of the second trench; a first dielectric layer that fills the third trench; a metal gate, located in the second trench and the first trench on the first dielectric layer.