A semiconductor memory system and an operating method thereof include: a memory device; and a memory controller including a processor, coupled to the memory device, containing instructions executed by the processor, and configured to provide sets of throttling numbers, select a throttling mode, calculate a garbage collection (GC)/HOST ratio based on at least a part of invalid count of garbage collection (GC) blocks and valid count of BGC blocks, and adjust throttling parameters of commands in accordance with the GC/HOST ratio and a number of erased blocks.