A method for a video encoder or decoder using an Intra Block Copy (IntraBC) mode is disclosed. If IntraBC mode is enabled and no loop filter is used for a current picture, a currently decoded reference picture for the current picture is marked as a long-term reference picture before and during decoding of the current picture. The currently decoded reference picture is assigned in reference picture list construction for IntraBC reference of the current picture and buffer fullness for the DPB is calculated before starting decoding the current picture by taking into account of only one reference picture buffer. According to another method, if the maximum DPB size is 1, a variable or a syntax element is set to a target value to impose restriction to use only one picture storage buffer in the DPB to store reference picture for encoding or decoding of a current picture.