Methods and apparatuses for generating a protection window signal for use in header detection of an optical storage medium are disclosed. One of the methods involves: extracting a maximum level of an RF-sum signal derived from the optical storage medium to generate a first level value; extracting at least one of DC level and bottom level of the RF-sum signal to generate a corresponding second level value; calculating a reference level according to the first and second level values; and comparing the RF-sum signal with the reference level to determine a protection window signal. When the accessing of the optical storage medium is switched from a first area with a first reflectivity to a second area with a second reflectivity, the response speed of extracting the DC level/bottom level of the RF-sum signal, the weighting of the second level value, or the DC level of the RF-sum signal is adjusted.