Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) single crystal phosphor (SCP) exhibits high internal quantum efficiency (IQE) and excellent thermal conductivity and stability. Such properties are promising in high-power laser-excited white lighting applications. However, YAG:Ce³⁺ SCP usually shows low luminous flux and color rendering index (CRI, Ra) when excited by a transmissive laser. In this study, a self-designed three-integrating sphere system was established to characerize the optical performances of YAG:Ce³⁺ SCP palates under both the reflective and transmissive 455 nm blue laser excitations. Next, its luminous and color rendering properties under transmissive laser excitation were improved by surface roughening treatment and sintering of a CaAlSiN₃:Eu²⁺ (CASN:Eu²⁺) phosphor-in-borate glass (PiBG) layer. The results revealed that: (1) When the YAG:Ce³⁺ SCP was excited by blue laser, its forward luminous flux was considerably lower than the backward one; (2) Under transmissive excitation, its luminous flux rolled up with the maximum increase rate of 109.19% when the averaged roughness (R_a) increased from 0.35 to 5.40 μm; (3) After sintering the CASN:Eu²⁺ PiBG layer on the roughening YAG:Ce³⁺ SCP, its luminous flux, CRI increased by 31.48% and 117.14%, and its color fidelity index (CFI, R_f) reached up to 82 under the transmissive 3.03 W blue laser excitation.