The design and performance analysis of a new sensor is introduced which is on board a small unmanned aerial vehicle (UAV) for coastal water remote sensing. The top level requirements of sensor are to have at least 4cm spatial resolution at 500m operating height, and 4° field of view (FOV) and 100 signal-to-noise ratio (SNR) value at 660nm. We determined the design requirements that its entrance pupil diameter is 70mm, and F-ratio is 5.0 as an optical design requirement. The three-mirror system is designed including aspheric primary and secondary mirrors, which optical performance are 1/15λ RMS wavefront error and 0.75 MTF value at 660nm. Considering the manufacturing and assembling phase, we performed the sensitivity, tolerance, and stray-light analysis. From these analysis we confirmed this optical system, which is having 4cm spatial resolution at 500m operating height, will be applied with remote sensing researches.