Due to the limitation of modeling of HVDC control loops in small signal stability analysis， current works on HVDC modulation controller design strongly depend on time-domain simulation， which cannot give more insight into mechanism of controllers. The paper induces a novel HVDC modulation loop model through customized control function of NEVA. By using the root locus method， different impacts of three-HVDC modulation control of China Southern Power Grid（CSG） with wide-area feedback signals on CSG's oscillation modes are analyzed. A strong nonlinear modal interaction phenomenon called‘modal resonance' is observed， and modal resonance may result in deterioration of damping. Finally， a scheme of coordination of multiple HVDC modulation to mitigate modal resonance is proposed， which is validated by NETOMAC time-domain simulation.