A child safety seat can be formed in a single body, and comprises a seating portion having an upper surface and opposite bottom surface, and two armrests projecting upward at left and right sides of the seating portion, the upper surface having left and right front corners provided with lobe portions, and the bottom surface having recessed portions respectively corresponding to the locations of the lobe portions. Each of the recessed portions has a shape that can fit with the shape of each of the lobe portions. When two instances of the seats are stacked on each other, the lobe portions of the lower seat are respectively fitted at least partially into the recessed portions of the upper seat, and the tapered inner surfaces of the armrests on the lower seat are in contact with the tapered outer surfaces of the armrests of the upper seat.