Abstract: A lumped model for flow instability in parallel channels is built for parallel rectangular channels and the influence of asymmetric condition on flow instability is analyzed theoretically in present work. The results show that the asymmetric conditions play a significant influence on flow instability in parallel channels: by keeping the average throttling constant, the influence of asymmetric throttling increases with the decrease of pressure or with the increase of mass flux; the threshold power is first increase and then decrease with the increase of asymmetric heating, and the influence of asymmetric heating increases with the increase of system pressure while decreases with the increase of inlet subcooling or mass flux.