Harvesting energy from ambient vibration sources with ultrathin flexible piezoelectric energy harvesters (PEHs) for battery-free electronics has received attention in recent years. However, the excitation modes in the environment and human body motion are more complicated than the ideal harmonic modes employed in previous theoretical analyses, and their influence on the efficiency of PEHs has received little attention. In this study, these environmental excitation modes are classified into three types, i.e., the triangular, sinusoidal, and square wave modes, with varied duty ratios. We derived theoretically the output power of flexible PEHs under these excitation modes and establish a simple scaling law, in which the normalized output power depends only on two combined normalized parameters, i.e., the intrinsic system parameter and the excitation mode. Results reveal that the output power of PEHs changes dramatically for different excitation modes with varied duty ratios even when all the other parameters including excitation amplitude, excitation frequency, electrical parameter, and geometrical and material parameters of the PEHs are identical. This paper may provide a systematic understanding in the effect of excitation modes on the output power of flexible PEHs and promote the realization of energy harvesting from the complex environmental and human body motions.