On the basis of pumping-power-dependent emission property, single-chip zinc-doped cadmium sulfide nanobelts are developed to monitor injected laser power and detect the profile of laser focal spots visually in the micron region through color changes. By doping zinc into cadmium sulfide nanobelts, band-edge emission with green color and deep-trap-state emission with red color are generated by injected laser beam pumping. The monotonic intensity ratio changes of these two emissions reflect the pumping power variations, so that the laser power and focus spot distribution can be monitored quantitatively by the on-chip nanobelts. Utilizing the on-chip zinc-doped cadmium sulfide nanobelts to detect input laser power, more than two thousand cycles of pumping laser power increasing and decreasing periodically are applied on the nanomaterial chip, accompanied by emission-color switching between yellow emission and green emission, which confirm that the zinc-doped cadmium sulfide nanobelts are stable and effective. The method involves visually perceiving the change of laser power and detecting the distribution profile of the laser spot. This optical ratiometric method based on nanostructures is quite different from the traditional detection mode by the thermoelectric effect. Therefore, it has potential application as a laser power monitoring tool for optically integrated micro-circuits.