The invention provides a CMOS image sensor and a forming method thereof, and a method for forming semiconductor devices, wherein the method for forming the CMOS image sensor comprises: providing a semiconductor substrate comprising a peripheral circuit area and a pixel unit area; forming a first pad oxide layer on the semiconductor substrate; removing the first pad oxide layer in the peripheral circuit area; forming a second pad oxide layer on the first pad oxide layer and the semiconductor substrate; etching the second pad oxide layer and the semiconductor substrate in the peripheral circuitarea, as well as the second pad oxide layer, the first pad oxide layer and the semiconductor substrate in the pixel unit area, forming a first shallow groove in the peripheral circuit area and forming a second shallow groove in the pixel unit area, wherein the first shallow groove is deeper than the second shallow groove; removing the second pad oxide layer and the first pad oxide layer after filling the first shallow groove and the second shallow groove with insulating oxide layers; and forming transistors on the semiconductor substrate in the peripheral circuit area and the pixel unit area.The invention has the advantage of ensuring the isolation function of the shallow grooves.