In my talk at the 2nd Galileo-Xu Meeting, I presented several different topics in 21cm cosmology for which I have done research. These includes the 21cm signature of the first stars[1,2], the 21cm signal from the IGM and minihalos[3], effect of dark matter annihila- tions on 21cm signal[4], the 21cm forest by ionized/neutral region[5], and the 21cm forest by minihalo and earliest galaxies[6,7]. In this conference proceeding I shall not repeat these discussions, but instead focus on the last part of my talk, i.e. the Tianlai project, an experiment effort on low redshift 21cm intensity mapping observation for dark energy measurements.