Phosphor‐converted white light‐emitting diodes (WLEDs) are extensively spread to every part of our daily life and trigger urgent needs for discovery of phosphors without cyan gap (spectral missing in the 480–520 nm range) in pursuit of “human‐centric” sunlight‐like full‐spectrum lighting. Here, a new ultra‐broadband green‐emitting K 3 La(PO 4 ) 2 :Eu 2+ phosphor is reported to meet this challenge. An efficient and positive double‐heterovalent substitution strategy is proved in K 3 La(Ca/Sr/Ba)(PO 4 ) 2 :Eu 2+ by the evidence of enhanced quantum yield, improved thermal stability, and the tunable luminescence color. The luminescence mechanisms are revealed in this paper. Fabricating a dichromatic 370 nm‐UV chip pumped WLED using K 3 La(Ca)(PO 4 ) 2 :Eu 2+ along with a commercially available red phosphor shows a continuity spectrum comparable to the solar spectrum with excellent CIE color coordinates of (0.350, 0.347), low color‐correlated temperature of 4853 K, and high color rendering index (Ra) of 96. It is shown that the double‐heterovalent substitution contributes to designing and optimizing phosphors for full‐spectrum WLEDs.