Abstract A suitable host material is pivotal for efficient and stable deep-blue phosphorescent organic light-emitting diodes (PhOLEDs). Here, we construct a deuterated exciplex-forming host with improved molecular stability and charge transport and firstly unveil an “external deuteration effect” on dopant, which reduces the shoulder emissions for slightly blue-shifted colours and also accelerates the radiative decay rates for improved photoluminescence efficiency. The corresponding deep-blue PhOLEDs based on two platinum complexes, PtON-TBBI and PtON-tb-DTB, achieve lower operational voltages and higher maximum external quantum efficiencies of 27.4/19.9% and power efficiency of 41.2/33.6 lm/W, respectively, compared to the hydrogen-based counterparts. Moreover, lifetimes of 370 and 557 h to reach 90% of the initial luminance of 1000 cd/m2 with Commission Internationale de l’Eclairage coordinates of (0.148, 0.165) and (0.153, 0.213) are achieved, 1.6 and 1.4 times longer than the ones based on the non-deuterated hosts with even blue-shifted colours.