A multi-channel memory storage device, a memory control circuit unit, and a data reading method are provided. The method includes: determining whether a storage space of a buffer memory is insufficient when a multi-channel access is performed; issuing a data read command corresponding to each of a plurality of multi-channels to a rewritable non-volatile memory module according to a logical address in a host read command in response to insufficient storage space of the buffer memory to read data corresponding to each of the plurality of multi-channels from a data storage area to a data cache area via the plurality of multi-channels; and allocating the storage space of the buffer memory to the rewritable non-volatile memory module after the storage space of the buffer memory is released and issuing a cache read command to move first data in data temporarily stored in the data cache area to the buffer memory.