The mechanisms for energy transfer including Förster resonance energy transfer (FRET) and radiative energy transfer in ternary-emissive system consists of blended-quantum dots (QDs, red-QDs blended with blue-QDs) emissive layer (EML) and blue-emissive hole-transport material that contained in quantum dot light-emitting diodes (QLEDs) are complicated. As the energy transfer could exhibit either positive or negative impact on QD's photoluminescence (PL) and electroluminescence (EL), it is important to analyze and modulate energy transfer in such ternary-emissive system to obtain high-efficiency QLEDs. In this work, we have demonstrated that proper B-QDs doping has a positive impact on R-QDs' PL and EL, where these improvements were attributed to the B-QDs' spacing effect on R-QDs which weakens homogeneous FRET among R-QDs and near 100% efficient heterogeneous FRET from B-QDs to R-QDs. With optimization based on the analysis of energy transfer, the PL quantum yield of blended-QDs (with R:B blending ratio of 90:10, in quality) film has been enhanced by 35% compared with that of unblended R-QDs film. Moreover, thanks to the spacing effect and high-efficiency FRET from B-QDs to R-QDs, the external quantum efficiency of QLEDs that integrate optimized blended-QDs (R:B=90:10) EML reaches 22.1%, which is 15% higher than that of the control sample (19.2%) with unblended R-QDs EML. This work provides a systematically analytical method to study the energy transfer in ternary-emissive system, and gives a valid reference for the analysis and development of the emerging QLEDs that with blended-QDs EML.