Slug flow is one of the most common flow patterns in horizontal gas-liquid two-phase flow. Structure detection of slug flow is of great significance for understanding the heat and mass transfer between phases and uncovering the mechanism of flow pattern transition. However, it is remarkably difficult to accurately measure the interface structures of slug flow with a single-sensing mode due to its multiscale characteristics. In this article, we design a combined ultrasonic and conductance sensing system that consists of a transmission ultrasonic transducer (TUT), a reflection ultrasonic transducer (RUT) and two parallel-wire conductance sensors (PWCSs). The response signals of the ultrasonic transducers and the conductance sensor for slug flows were collected in an experiment of horizontal gas-liquid flow. Based on the different measurement mechanisms of the ultrasonic transducers and the conductance sensor, the 2-D gas-liquid interfaces in slug flows were reconstructed. The experimental results demonstrate that, compared with the TUT and the PWCS, the RUT has obvious advantages in the detection of the local gas-liquid interface structure. In addition, based on the cross-correlation velocity measured by the PWCSs, the slug lengths in the gas-liquid flow were accessed. It was found that the slug lengths derived from the ultrasonic transducers and the conductance sensor indicate a good agreement and present complicated evolution characteristics due to the coalescence and crushing of the slugs.