Disclosed are a modified polyester fiber and a process for producing the same. The modified polyester fiber comprises the component of alkoxylated 2-methyl-1,3-propanediol and has the properties of easy dyeability at low temperatures and good colorfastness to laundering. The modified polyester fiber can be produced by undergoing a polymerization reaction of alkoxylated 2-methyl-1,3-propanediol, alkanediol, and terephthalic acid or an alkyl ester thereof, and then spinning. The modified polyester fiber can be dyed at low temperature and has excellent dyeability and colorfastness to laundering. Therefore, this modified polyester fiber can be widely used in the application of synthetic fiber product.