Two-phase flow widely exists in the process industry, where ultrasonic tomography provides reliable visualization with benefits of noninvasion, nonradiation, and low-cost. To provide real-time reconstruction of low acoustic impedance contrast medium distribution existed in two-phase flow, a novel approach using continuous-wave excitation is proposed. With the basic measurement model introduced, a subspace filtered reconstruction algorithm is introduced with improvements on both accuracy and speed. Through nonuniform shape constrained range filter, the relative image error (RE) is reduced by 32.94% and 33.03% compared to simultaneous algebraic reconstruction technique (SART) and filter back projection (FBP) methods. Through derivation of offline iteration/online reconstruction (OIOR) method and truncated singular value decomposition (TSVD), the computing speed increases to 122 frame/s, which fits the requirement of online monitoring. A prototype of the measurement system is introduced with improved design of bipolar high-voltage excitation module and fan-beam angle piezoelectric transducers. Qualitative and quantitative results show that the proposed method has better image accuracy compared to conventional methods.