To meet the demands of laser ion acceleration at a high repetition rate, we have developed a comprehensive diagnostic system for real-time and in-situ monitoring of liquid sheet targets (LST). The spatially resolved rapid characterizations of a LST’s thickness, flatness, tilt angle and position are fulfilled by different subsystems with high accuracy. With the help of the diagnostic system, we reveal the dependence of thickness distribution on collision parameters and report the 238-nm liquid sheet generated by the collision of two liquid jets. The control methods for the flatness and tilt angle of LSTs have also been provided, which are essential for applications of laser-driven ion acceleration and others.