In step and scan lithographic tools whose laser sources are excimer lasers, an energy detector is needed to monitor excimer laser pulse energy in real-time and accurately so as to realize precise exposure dose control. An energy detector, which is composed by focusing lens, an integration rod, a special crystal, a filter, a photodiode and a signal processing circuit, is designed. Specially, the second-order active low-pass filter is used for narrow pulse shaping so as to reduce the difficulty of the signal processing circuit. Also, an experimental setup is designed to testing the performances of the energy detector. The results show that the relative error between the measured value and the designed value of the shaped pulse for the energy detector is about 3% , and the response time of the energy detector can meet the application of pulse energy measurement with pulse′ s repetition rate up to 4 kHz. Secondly, with a double-beam ratiometric method which is used to remove excimer laser pulse energy fluctuation, the performances of the energy detector are tested; the energy detector′s repeatability is 0.26%; within a dynamic range of pulse energy from 0.1 μJ to 1 μJ per pulse, the linear fit is V=0.59841E-0.01508 and R2=99.976% . Experimental results prove that the secondorder active low-pass filter can be used for pulse shaping; the repeatability and the linearity of the designed energy detector are perfect, and the energy detector can be used to accurately measure the energy of high repetition rate pulse for excimer lasers in lithographic tools.