The 21-cm intensity mapping technique is a novel method of probing the dark energy and the growth of structure. Without measuring the emission from each individual galaxy, the intensity mapping technique aims to probe a large volume of Baryon Acoustic Oscillation samples which can accurately map out the cosmic evolution in low-redshift Universe. In this talk, I will present the Fisher-matrix forecast that we have done for BINGO, FAST and SKA radio telescope. In addition, we will discuss the full treatment of the relativistic correction for the 21-cm intensity field, and show the prospects of separating redshift-space distortion, integrated Sachs-Wofe effect from the main intensity fluctuations.