To improve the power factor of thyristor rectifiers which are still popular in high power rates， a novel multi-pulse thyristor rectifier is proposed. Two three-phase thyristor bridges can operate in both parallel and series modes by using an auxiliary circuit with two diodes and a switch. The flexible topology is to avoid the deep angle triggering of the thyristors in light load， thus a higher power factor and lower harmonic contents can be achieved. This novel peak current controller achieves a seamless switching transition during topology changing and a better current tracking capability. Based on the periodical voltage-second integral calculation on DC side inductor， the controller compares the sum of current sampling and the expected rising current caused by voltage-second integral on inductor in real time to trigger the thyristor. Simulations and experiments show the seamless transition during topology switching. Details of power quality improvement under different loads are listed and compared with a traditional 12-pulse converter.