A selecting bad data column method suitable for a data storage device is provided. The data storage device includes a control unit and a data storage medium. The selecting method performed by the control unit includes: reading written data of each data column as read data; comparing the read data and the written data of each data column to calculate an average number of error bits of each data column; determining whether the average number of error bits of each data column is greater than or equal to a predetermined value; and recording a data column as a bad data column when the average number of error bits of the data column is greater than or equal to the predetermined value. In this way, in order to avoid the problems that the error correction code can't be corrected or the correction capability is excessively consumed.