In order to meet the needs of multi-parameter variable evaluation of composite materials under complex loads in nondestructive testing, a dual-function digital speckle pattern interferometry system based on optical multiplexing is proposed to realize the measurement functions of both digital speckle pattern interferometry and digital shearography. By controlling one of the combinations of a mirror and quarter-wave plate, the optical setup of digital speckle pattern interferometry is formed when the combination is in the first position, allowing the out-of-plane displacement to be measured. The optical setup of digital shearography is formed to realize measurement of slope when the combination is in the second position. During the process of measurement, the out-of-plane displacement and slope of the object surface due to a single load can be simultaneously measured by simply switching the position of the combination. The optical setup of the dual-function digital speckle pattern interferometry enjoys the advantages of simple structure and high switching efficiency. High quality measurement results of displacement and slope can be obtained using the dual-function digital speckle pattern interferometry. It has been proved experimentally that the dual-function digital speckle pattern interferometry is suitable for field use in nondestructive testing of composite materials due to its superior performance in anti-interference and high-sensitivity testing.