Quantitative phase microscopy, as a non-destructive and non-invasive measurement technique, can indirectly reflect three-dimensional (3D) morphology and optical properties of transparent microstructure object by measuring phase information. In recent years, this kind of technique has been widely used to detect and investigate the characteristics of biological cells and it has become more and more important in the field of modern biomedical and life science. In this paper, only by using a single cube beamsplitter interferometer, a simple single-shot dual-channel quantitative phase microscopic measurement technique is demonstrated for 3D quantitative phase imaging of biological cells. In the proposed method, a conventional non-polarized cube beamsplitter is the most pivotal element. Unlike its traditional application method, the cube beamsplitter is tilted in a nonconventional configuration and the illumination beam is only incident on the left (or right) half of the cube beamsplitter (just the one side of central semi-reflecting layer), and a very small angle is introduced between the central semi-reflecting layer and the optical axis of incident beam. Based on the light splitting characteristic of the cube beamsplitter, two replicas of incident beam are generated. These two generated replicas (transmission beam and reflection beam) are of symmetry with respect to each other, and they will encounter and form interference when the direction of the incident beam meets a certain condition. Adjust the sample to a suitable position and make it only contact one half of incident beam, and the modulated beam will be seen as the object beam and the remaining clean half of incident beam as the reference beam. When the interference phenomenon occurs, two interference channels with a relative π (rad) phase-shift in one interferogram are acquired simultaneously only using one digital camera, and the higher spatial frequency of interference fringes can be achieved by adjusting a relatively big angle between the central semi-reflecting layer and the optical axis of incident beam. Because of the off-axis interference mode, we only need to record one interferogram to gain the continuous phase information and avoid using complex phase-shift techniques. At the same time, this proposed method is of simple structure and easy to operate due to using less ordinary off-the-shelf optical elements. All these simplify the structure of the system and reduce the cost of the system as much as possible. Finally, the phase information of paramecium is successfully obtained from different interference channels respectively. Furthermore, according to the characteristic of π (rad) phase-shift, we also realize the calibration and determination of ultimate precise phase information of sample by using the method of averaging between these two channels. The experimental results show that our proposed method is suitable for 3D surface morphology measurement of small transparent samples.