Signal source is a key component in both transmitter and receiver of a radar system, and its property determines the performance of the radar system. The signal generation based on electronic technologies is limited in aspects of carrier frequency, signal bandwidth, and noise properties, which cannot meet the requirement for signals with high frequency, large bandwidth and low noise. Microwave photonics, as a newly arisen and promising technology, can generate high quality microwave signals and would find wide applications in future radar systems. The recent progress of radar signal generation by microwave photonic technologies is presented. The generation of high quality local oscillator signal based on optoelectronic oscillators, and the generation of linear chirped or phase coded microwave signals are discussed. Through the analysis of the above techniques, it is found that microwave photonic signal generation, with the advantages of wide bandwidth and immunity from interference, is one of the efficient ways to break through the bottleneck for current radar signal generation.