Bound states in the continuum (BIC) has made significant progress in photonic integrated circuits, but there are still some limitations in practical applications. When the mode deviates from the BIC, its Q value decays rapidly. These can limit the performance of BIC under wide-angle incidence. The application limitation of BIC is mainly because all types of BIC are discrete mode points in the k - space. This kind of point-like BIC or quasi-BIC, on the one hand, has extremely high requirements for the incident angle: even a slight deviation can cause a sharp drop in the Q factor. On the other hand, it is also extremely sensitive to the geometric parameters of the structure such as the size, position and offset of the holes. Even the slightest manufacturing error can cause the formant position to shift and the Q value to drop sharply. Therefore, if we can construct a BIC in a continuous k - space, we can largely remove the constraints on the application of traditional BIC. In this work, we design a simple photonic crystal slab and calculate and analyze its band structure and quality factor. By optimizing the structural parameters, a square-shaped quasi-BIC loop with tunable side length is identified in k - space. Based on the relationship between the quasi-BIC loop and the equifrequency contours, as well as the characteristics of the mode field distribution, it is revealed that this square-shaped quasi-BIC originates from the effect of total internal reflection and standing-wave resonances in the structure. The existence of the square-shaped quasi-BIC loop is further confirmed by the Fano spectral line with high quality factors at a special incident angle or frequency which corresponds to the position of the quasi-BIC loop. The square-shaped quasi-BIC loop provides a large angle-bandwidth response, thereby broadening the practical application scope of BIC.