Abstract: Fluctuations of toxicant concentions in long-term continuous-flow tests are mainly due to malfunction of the toxicant delivery system. Toxican analysis are essential in improving data quality. To reduce toxicant fluctuations, a serial diluter was modified and Masterflex metering pumps which delivered toxicant solution and dilution water were used. The continuous-flow test system with RTMS Which included Masterflex pump, pour-through cell installation, spectrometer and recorder ensured a high accuracy. In a 56-day test, The RTMS provided real-time information on the diluter. The variahon coefficients of toxicant concentration ranged from 2.16% to 6.13%. The successful operation of the system provided technical ensurance for intermediate-term and long-term ecotoxicological test.