Adaptive power loading algorithms for variable beam variable rate (VBVR) Bell Laboratories Layered Space?Time (BLAST) system under QoS constraints are proposed. The truncated right unitary matrix as precoder at transmitter and truncated left unitary matrix as decoder at receiver which are obtained by singular value decomposition (SVD) choose variable eigen-subchannels to carry BLAST codes with variable layers. We compare the average spectral efficiency (ASE) of several power allocation algorithms continuous rate (CR) system, one-time power loading (OPL) system, variable beam variable rate system under instantaneous power constraint (VBVR-IPC) and variable beam variable rate system under average power constraint (VBVR-APC). Simulation shows that VBVR-APC and VBVR-IPC perform better than conventional OPL algorithm.