Accurate knowledge of multi-beam radiation patterns of millimeter-wave (mmWave) phased arrays is key to mmWave radio system performance. The multi-beam radiation patterns can be measured by conventional antenna test ranges in a sequential manner. However, these tasks can be time-consuming and low-efficient in practice, especially for large-scale arrays with many beam-steering states. To address this problem, this article proposes a multi-beam reconstruction method, which can measure all possible steered beams in a fast and efficient manner. In the proposed method, multiple beam patterns are reconstructed synchronously by a fast element pattern retrieving method based on a few sparse measurement samples, which were obtained in a multi-probe setup or with the help of a turntable in a single probe setup. The proposed method is experimentally validated in a compact antenna testing range (CATR) setup at 28 GHz for a 4 × 4 mmWave phased array with an integrated antenna-in-package (AiP) design. The experimental results show that the reconstructed array multi-beam patterns by the proposed method are comparable to the conventional CATR measurement results but in a faster way, which validates the effectiveness of the proposed method.