Optical tweezers is one of the commonly used technologies to research on protein force spectroscopy. However, whether optical tweezers system has the capability of force spectroscopy measurement at the molecular scale is vital to single molecule experiments. In this paper, we test the capability of our home-built dual-trap optical tweezers system by stretching polyprotein (NuG2)<sub>8</sub> which is made of eight identical tandem repeats of NuG2. With the constant velocity stretching and relaxation mode, we achieve a lot of experimental data and get the contour length increment of (NuG2)<sub>8</sub> rapidly from the unfolding processes after fitting these data. The result is consistent with existing reports, which demonstrates optical tweezers system has the force spectroscopy test ability and (NuG2)<sub>8</sub> can be used as a new standard sample to evaluate the test performance of optical tweezers. Using polyprotein (NuG2)<sub>8</sub> as standard sample has two advantages: stretching polyprotein can help improve the efficiency of data statistics and a large number of experiments can reduce the randomness of the system when testing.