Split-gate flash memory and forming method thereof are provided. The method includes: forming a first dielectric layer on a semiconductor substrate; forming a floating gate layer on the first dielectric layer; forming a mask layer on the floating gate layer; etching the mask layer until first groove exposing the floating gate layer is formed; forming a protective sidewall on sidewall of the first groove; forming a gate dielectric layer on bottom and the sidewall of the first groove; forming two control gates on the gate dielectric layer, the remained first groove serving as second groove; etching the gate dielectric layer and the floating gate layer at bottom of the second groove until third groove exposing the first dielectric layer is formed; forming a source in the semiconductor substrate under the third groove; and forming a second dielectric layer in the third groove. Reliability and durability of the memory are improved.