I will introduce our recent work on excitation and radiation model of atomic hydrogen in ionized gas, as well on-going radio recombination line (RRL) observations with FAST. The key result is that most RRLs are weak maser instead of thermal emission. We also discuss the systematic bias of using line (RRL) to continuum ratio (free-free) to obtain electron density of ionized gas under LTE assumption. A preliminary result of L band RRL observation with the FAST will also be present.