Transient electronics that disintegrate via material dissolution or depolymerization under certain stimuli have great potential in biomedical and military applications. The triboelectric nanogenerator (TENG), an emerging mechanical energy harvesting technology with great flexibility in material choices, is promising in offering transient power sources. Previously reported transient energy harvesters using biodegradable polymers require solution-based degradation and have limited applications in non-biological scenarios. A short time span, sunlight-triggered degradable TENG is developed. The main substrate includes an acid-sensitive poly(phthalaldehyde) (PPHA), a photoacid generator (PAG), and a photosensitizer (PS). Through photo-induced electron transfer, the ultraviolet radiation absorbed by the PS is transferred to the PAG to generate photoacids that trigger the depolymerization of PPHA. Transient TENG-based mechanical energy harvesters and touch/acoustic sensors are successfully demonstrated by embedding silver nanowires onto the PPHA-based films. The fabricated devices degrade rapidly under winter sunlight. The degradation rate can be further tuned via changing the ratio of photosensitive agents. This work not only broadens the applicability of TENG as transient power sources and sensors, but also extends the use of transient functional polymers toward advanced energy and sensing applications.