Fused silica is widely used as a key optical material in deep-ultraviolet exposure and high-power laser systems owing to its broad spectral transmission, high damage threshold, and excellent stability. However, under high-energy pulsed irradiation at 193 nm, it can still exhibit degradation phenomena such as transmittance drift, enhanced scattering, and surface damage, making it urgent to quantitatively evaluate its optical stability under conditions close to actual operation. To address this, we construct an in situ detection system for fused silica based on a 193 nm excimer laser. The system integrates a laser collimation optical path, a homogenization optical path, a dual-channel optical power monitoring path based on magnesium fluoride beam splitter, a three-axis motorized scanning stage, and a red-light auxiliary alignment optical path, alongside an integrated hardware and software platform for equipment control, environmental regulation, and data analysis. By utilizing power normalization and filtering, change-point detection and trend fitting, as well as spatial statistical sampling, the system enables real-time tracking of the transmittance evolution and damage behavior of fused silica under 193 nm irradiation, supporting single-point successive, multi-point sequential, and long-term continuous irradiation tests. Experimental results demonstrate that under a single-pulse energy of 8 mJ, a repetition rate of 1 000 Hz, and a single-point irradiation time of approximately 30 min, the system can effectively suppress measurement drifts caused by laser preheating and thermal accumulation. Cross-validation with a spectrophotometer shows that the maximum and average errors for single-point transmittance degradation are 0.29% and 0.12%, respectively. Spatial scanning tests on a 160 mm×160 mm large-area sample further show that the average transmittance attenuation of the sample is 4.04% and that the in-plane distribution has good macroscopic consistency. This system provides a reliable in situ measurement method for design optimization, lifetime assessment, and engineering screening of deep-ultraviolet optical materials.