A power-off period estimating method for a solid state storage device is provided. A memory array of a non-volatile memory of the solid state storage device includes plural blocks. Firstly, a first quality parameter of a first block of the plural blocks is calculated before the solid state storage device is powered off. When the first block is corrected at a first time counting value, a first read voltage set of the first block is acquired and the first time counting value is recorded. Then, the first block is corrected after the solid state storage device is powered on, so that a second read voltage set of the first block is acquired. Then, a power-off period is calculated according to the first quality parameter, the first read voltage set, the second read voltage set and the first time counting value.