In this study, a novel concept of 3D constant life diagram (CLD) for fatigue life prediction of a unidirectional (UD) polymer composite under spectrum load is proposed. Further, it is constructed and used to predict the fatigue life of a UD carbon fiber composite (CFC) subjected to a standard spectrum load sequence at an arbitrary off-axis angle. First, UD IMA/M21 CFC laminates were fabricated by an autoclave process. Static mechanical tests were conducted to determine the tensile and compressive strength at various off-axis angles ranging from 0° to 90°. Then the constant amplitude (CA) fatigue tests at three different stress ratios, R = σmin/σmax of 0.1 (tension-tension), −1.0 (tension-compression), and 10.0 (compression-compression) and at various off-axis angles between 0° and 90° at each of these stress ratios were performed to determine stress-life curves. Using the static and CA fatigue data generated, 3D CLD for UD CFC was constructed. Further, fatigue life of UD CFC subjected to a standard mini-FALSTAFF spectrum load sequence at an arbitrary off-axis angle of 20° was predicted following an empirical method using 3D CLD. Fatigue tests under the mini-FALSTAFF spectrum load sequence at an off-axis angle of 20° were also conducted at various reference stresses and compared with the predictions. A reasonably good correlation was observed between the predicted and experimental fatigue lives under off-axis spectrum loads.