The present invention provides an LED light string color mixing and synchronization circuit, in which each of LED lights composed of an LED light string includes two input terminals and two output terminals. The two input terminals are respectively an input terminal of reverse parallel connection of any two of three primary light-emitting chips of R, G, B and an anode input terminal of the remaining primary light-emitting chip and the two output terminals are respectively an output terminal of reverse parallel connection of any two of three primary light-emitting chips of R, G, B and a cathode output terminal of the remaining primary light-emitting chip. Thus, each of the LED lights of the LED light string is connected in a two-input two-output series connection to form a light string arrangement with two terminals thereof in combination with four electrical wires.