Bioluminescence, due to its high sensitivity, has been exploited in various analytical and imaging applications. In this work, we report a highly stable, cell-transductable, and wavelength-tunable bioluminescence system achieved with an elegant and simple design. Using aqueous in situ polymerization on a bioluminescent enzyme anchored with polymerizable vinyl groups, we obtained nanosized core-shell nanocapsules with the enzyme as the core and a cross-linked thin polymer net as the shell. These nanocapsules possess greatly enhanced stability, retained bioactivity, and a readily engineered surface. In particular, by incorporating polymerizable amines in the polymerization, we endowed the nanocapsules with efficient cell-transduction and sufficient conjugation sites for follow-up modification. Following in situ polymerization, decorating the polymer shell with fluorescent quantum dots allowed us to access a continuous tunable wavelength, which extends the application of such bioluminescent nanocapsules, especially in deep tissue. In addition, the unique core-shell structure and adequate conjugation sites on surface enabled us to maximize the BRET efficiency by adjusting the QD/enzyme conjugation ratio.