The evaluation of urban open spaces plays an important role in landscape design, urban management and public security. However, few three-dimensional openness approaches with high accuracy and applicability to multiple data models have been developed. A method has been developed to measure the openness of an arbitrary location in an urban space accurately. This method is based mainly on the geometric subdivision of 3D space. First, a complete visual sphere was constructed with the viewpoint as the centre. The visual sphere was subsequently uniformly subdivided by multiple lines of sight at certain horizontal and vertical angle intervals, generating multiple discrete visual difference elements (VDEs). Finally, the specific visible volumes in each VDE were calculated separately to accumulate the all-visible volumes. The visible space index (VSI) was defined as the ratio of all visible space volumes to the visual sphere. The results indicate that the proposed method can accurately calculate the volume of visible space at any location, and the corresponding VSI reflects the difference of visible space in urban environments under different terrain conditions. The proposed method is expected to provide technical support for large-scale urban openness mapping, consequently contributing to the unified evaluation of urban space.