The calibration of the ICARUS photo-detection system is based on a low power laser diode at 405 nm. Laser pulses arrive to one optical switch and then are sent to 36 UHV flanges, by 20 meters long optical patches. Light is then delivered to the ten PMTs connected to a single flange, by 7 m long injection optical patches. Extensive tests of the used components and care in the design of the optical system have guaranteed to each PMT a sizeable calibration signal with minimal distortion, with respect to the original one. Gain equalization of PMTs has reached a 1% resolution. In this procedure data from background photons were also used. The distribution of the PMTs’ signal arrival time has a distribution with a resolution less than 1 ns, thus allowing a good determination of the absolute event timing. The status of the laser calibration system with its possible upgrades will be reported.