Objective To observe the toxicological changes in Sprague-Dawley (SD) rats and Beagle dogs after continuous administration of the high molecular weight pharmaceutical excipient polyethylene glycol-polylactic acid copolymer (mPEG-PLA), explore possible toxic target organs or tissues and the reversibility of their damage, and provide a reference for evaluating the safety of this high molecular weight pharmaceutical excipient. Methods A total of 120 SD rats and 40 Beagle dogs, half male and half female, were randomly divided into a control group and low, medium, and high dose groups of the test substance. The self-made high molecular weight pharmaceutical excipient was administered by intravenous injection once a day for 90 consecutive days, followed by a 28-day recovery observation period after cessation of medication. The observed indicators included clinical observation, body weight, hematology, blood biochemistry, immune function, and pathological examination, etc. Results Clinical observations mainly showed that Beagles in the medium and high dose groups had swelling or scabs on multiple parts of the body, abdominal swelling, vomiting, reduced activity and diet, etc.; Beagles in the high dose group had increased body weight. Both strains of animals showed a dose-related decrease in peripheral blood monocyte count and percentage, a decrease in total protein and albumin, and an increase in total bilirubin and direct bilirubin; pathological changes related to medication included an increase in the weight of the liver and spleen, and a large number of foamy phagocytic cells were observed in the liver sinusoids, splenic red pulp, and lymph node medulla. In addition, Beagle dogs showed vacuolar changes in the mesangial cells of the glomeruli and proteinuria. Conclusion Under the conditions of this experiment, the high molecular weight pharmaceutical excipient polyethylene glycol-polylactic acid copolymer (mPEG-PLA) can cause defensive reactions such as enhanced phagocytic cell function in the liver, spleen, kidney, and lymph nodes of animals SD rats and Beagle dogs, which can lead to secondary protein loss, edema, and other secondary disease.