A rotor spinning method for four-sliver asynchronous inputting and three-level carding, comprising: 1) feeding fiber by four combined feeding rollers (2-5, 2-6, 2-7, 2-8) with four rotational freedom degrees into carding area where a three-level carding roller comprising three carding rollers (2-10, 2-11, 2-12) is used for carding; 2) moving the four combined feeding rollers (2-5, 2-6, 2-7, 2-8) at linear speeds V 01, V 02, V 03, V 04, respectively; moving a rotor at a linear speed V 4 and moving a guide roller (1-14, 1-15) at a linear speed V 5; setting linear densities of four slivers (2-1, 2-2, 2-3, 2-4) drafted by the four combined feeding rollers (2-5, 2-6, 2-7, 2-8) to be ρ 1, ρ 2, ρ 3, and ρ 4, respectively, and setting a rotor spun yarn density to be ρ, such that a draft ratio of a rotor spinning yarn is as below: E = ρ 1 + ρ 2 + ρ 3 + ρ 4 V 5 ρ 1 V 01 + ρ 2 V 02 + ρ 3 V 03 + ρ 4 V 04 = ρ 1 + ρ 2 + ρ 3 + ρ 4 ρ a linear density of the rotor spun yarn is formed, ρ = ρ 1 V 01 + ρ 2 V 02 + ρ 3 V 03 + ρ 4 V 04 V 5 wherein speeds of the three carding rollers (2-10, 2-11, 2-12) are as follows: a speed of a first carding roller is 1500-3000 rpm, a speed of a second carding roller is 3000-6000 rpm, a speed of a third carding roller is 6000-12000 rpm; 3) blending ratios of the four slivers (2-1, 2-2, 2-3, 2-4) in the rotor spinning yarn are K1, K2, K3, and K4, respectively: K 1 = ρ 1 V 01 ρ 1 V 01 + ρ 2 V 02 + ρ 3 V 03 + ρ 4 V 04 K 1 = ρ 2 V 02 ρ 1 V 01 + ρ 2 V 02 + ρ 3 V 03 + ρ 4 V 04 K 3 = ρ 3 V 03 ρ 1 V 01 + ρ 2 V 02 + ρ 3 V 03 + ρ 4 V 04 K 4 = ρ 4 V 04 ρ 1 V 01 + ρ 2 V 02 + ρ 3 V 03 + ρ 4 V 04 4) assuming that the speed V 5 of the guide roller is invariable, the variables of feeding speeds V 01, V 02, V 03, V 04 of the four combined feeding rollers (2-5, 2-6, 2-7, 2-8) of the four slivers (2-1, 2-2, 2-3, 2-4) are respectively as follows: V 01 ' = V 01 + ΔV 01, V 02 ' = V 02 + ΔV 02, V 03 ' = V 03 + ΔV 03, V 04 ' = V 04 + ΔV 04,, then the dynamic linear density of the rotor spun yarn is obtained according to formula Δρ = ρ 1 Δ V 01 + ρ 2 Δ V 02 + ρ 3 Δ V 03 + ρ 4 Δ V 04 V 5 5) assuming that ρ 1 = ρ 2 = ρ 3 = ρ 4 = ρ 0, V 01 +V 02 +V 03 +V 04 = V 0, obtaining a reference blending ratio according to the formulas,,, as K 1 = V 01 V 0, K 2 = V 02 V 0, K 3 = V 03 V 0, K 4 = V 04 V 0, wherein when V 01 +V 02 +V 03 +V 04 →V 01 +ΔV 01 +V 02 +ΔV 02 +V 03 +ΔV 03 +V 04 +ΔV 04, the blending ratio becomes as below: K 1 ′ = V 01 + Δ V 01 V 0 + Δ V 0 K 2 ′ = V 02 + Δ V 02 V 0 + Δ V 0 K 3 ′ = V 03 + Δ V 03 V 0 + Δ V 0 K 4 ′ = V 04 + Δ V 04 V 0 + Δ V 0 realizing dynamically adjustable spinning of different color blending ratios or color mixing ratios in the yarn with different fibers or colors by controlling V 01, V 02, V 03, and V 04. 1) feeding fiber by four combined feeding rollers (2-5, 2-6, 2-7, 2-8) with four rotational freedom degrees into carding area where a three-level carding roller comprising three carding rollers (2-10, 2-11, 2-12) is used for carding; 2) moving the four combined feeding rollers (2-5, 2-6, 2-7, 2-8) at linear speeds V 01, V 02, V 03, V 04, respectively; moving a rotor at a linear speed V 4 and moving a guide roller (1-14, 1-15) at a linear speed V 5; setting linear densities of four slivers (2-1, 2-2, 2-3, 2-4) drafted by the four combined feeding rollers (2-5, 2-6, 2-7, 2-8) to be ρ 1, ρ 2, ρ 3, and ρ 4, respectively, and setting a rotor spun yarn density to be ρ, such that a draft ratio of a rotor spinning yarn is as below: E = ρ 1 + ρ 2 + ρ 3 + ρ 4 V 5 ρ 1 V 01 + ρ 2 V 02 + ρ 3 V 03 + ρ 4 V 04 = ρ 1 + ρ 2 + ρ 3 + ρ 4 ρ a linear density of the rotor spun yarn is formed, ρ = ρ 1 V 01 + ρ 2 V 02 + ρ 3 V 03 + ρ 4 V 04 V 5 wherein speeds of the three carding rollers (2-10, 2-11, 2-12) are as follows: a speed of a first carding roller is 1500-3000 rpm, a speed of a second carding roller is 3000-6000 rpm, a speed of a third carding roller is 6000-12000 rpm; 3) blending ratios of the four slivers (2-1, 2-2, 2-3, 2-4) in the rotor spinning yarn are K1, K2, K3, and K4, respectively: K 1 = ρ 1 V 01 ρ 1 V 01 + ρ 2 V 02 + ρ 3 V 03 + ρ 4 V 04 K 1 = ρ 2 V 02 ρ 1 V 01 + ρ 2 V 02 + ρ 3 V 03 + ρ 4 V 04 K 3 = ρ 3 V 03 ρ 1 V 01 + ρ 2 V 02 + ρ 3 V 03 + ρ 4 V 04 K 4 = ρ 4 V 04 ρ 1 V 01 + ρ 2 V 02 + ρ 3 V 03 + ρ 4 V 04 4) assuming that the speed V 5 of the guide roller is invariable, the variables of feeding speeds V 01, V 02, V 03, V 04 of the four combined feeding rollers (2-5, 2-6, 2-7, 2-8) of the four slivers (2-1, 2-2, 2-3, 2-4) are respectively as follows: V 01 ' = V 01 + ΔV 01, V 02 ' = V 02 + ΔV 02, V 03 ' = V 03 + ΔV 03, V 04 ' = V 04 + ΔV 04,, then the dynamic linear density of the rotor spun yarn is obtained according to formula Δρ = ρ 1 Δ V 01 + ρ 2 Δ V 02 + ρ 3 Δ V 03 + ρ 4 Δ V 04 V 5 5) assuming that ρ 1 = ρ 2 = ρ 3 = ρ 4 = ρ 0, V 01 +V 02 +V 03 +V 04 = V 0, obtaining a reference blending ratio according to the formulas,,, as K 1 = V 01 V 0, K 2 = V 02 V 0, K 3 = V 03 V 0, K 4 = V 04 V 0, wherein when V 01 +V 02 +V 03 +V 04 →V 01 +ΔV 01 +V 02 +ΔV 02 +V 03 +ΔV 03 +V 04 +ΔV 04, the blending ratio becomes as below: K 1 ′ = V 01 + Δ V 01 V 0 + Δ V 0 K 2 ′ = V 02 + Δ V 02 V 0 + Δ V 0 K 3 ′ = V 03 + Δ V 03 V 0 + Δ V 0 K 4 ′ = V 04 + Δ V 04 V 0 + Δ V 0 realizing dynamically adjustable spinning of different color blending ratios or color mixing ratios in the yarn with different fibers or colors by controlling V 01, V 02, V 03, and V 04.