Objective To develop an instrument for testing the swimming condition of load-carrying animals. Methods Steel balls in different diameter, which were interchangeable, were used as loads of different weights. Experimental animals were allowed to swim with different loads in a water pool at constant temperature. Proximity sensors were used for signal conversion. A 51-Series microcontroller was used to control and record the experiment signals. Results A data-memory based instrument to test swimming conditions of animals carrying different load was developed. This instrument could be used to perform six parallel tests. It could automatically record swimming conditions of load-carrying animals. 36 mice were successfully tested with this instrument, demonstrating excellent results. Conclusion A data-memory based instrument to test swimming conditions of animals has been successfully developed, which offers an novel tool in studies to test swimming conditions of load-carrying animals.