We report an on-chip light-incorporated in situ transmission electron microscopy (LI2ST) approach for probing metal halide perovskites (MHPs) at the nanoscale, realizing the real-time, site-specific tracking of the light-triggered structure transformation. This in situ platform is based on a specifically designed microelectromechanical systems (MEMS) chip that offers the capability of light illumination with adjustable intensity and tailorable multiwavelength. The excellent operational reliability of the platform allows for the continuous observation of nanoscale regions of interest, recording the morphological and structural evolutions of perovskite grains and grain boundaries. A proof-of-concept demonstration shows a polycrystalline MHP film undergoing decomposition upon continuous light illumination. Counterintuitively, the decomposition starts and expands within the intragrain regions rather than at the grain boundaries. This work demonstrates an unprecedented ability to reveal light-triggered structural-phase variation for illuminating the dynamic behaviors of MHPs with implications for various energy applications.