The invention discloses a method for forming a groove, which is used for etching an insulating layer of a semiconductor device to form the groove, wherein the insulating layer consists of an etch stop layer, a low dielectric constant (Low-K) insulating material layer and a hard mask layer which are formed on a semiconductor substrate in sequence. The method comprises the following steps: coating a photoresist layer on the hard mask layer; patterning the photoresist layer; etching the hard mask layer and the Low-K insulating material layer in turn by using the patterned photoresist layer as a mask in an etching reaction cavity, stopping etching in the etch stop layer to form the groove; and performing in-situ ashing treatment on the photoresist layer in two steps in the same reaction cavity, wherein the first step is to dilute residual etching gas in the etching reaction cavity by adopting nitrogen (N2), and the second step is to carry out ashing by adopting the carbon monoxide (CO) or carbon dioxide (CO2). By adopting the method, the defect of facet top profile can be effectively overcome.