Three helicopter-slung load control systems were developed to study the effect of slung load on helicopter’s flying qualities, based on command filter system, PID control and dynamic inverse control. Control system were investigated using simulation and frequency domain analysis according to the requirements presented by ADS-33E and flying qualities criteria for rotorcraft with slung load. Subsequently, the effect of slung load mass and sling length on helicopter flying qualities was analyzed. The result showed that the load state feedback can improve helicopter’s handling qualities, the gain margin reached 126 dB and the bandwidth was 331 rad/s. The lagged cable angle feedback can damp the load swing effectively at the cost of getting the bandwidth down to 062 rad/s, and the helicopter with a long sling and heavy load may get poor flying qualities.