Above-threshold ionization (ATI) of the sodium atom is investigated in a Debye plasma environment using linearly polarized femtosecond laser pulses of intensity I= 1×10¹³ W/cm². ATI is studied in two-color laser fields by combining fundamental and harmonic frequencies ω–2ω (800–400 nm) and ω–3ω (800–266.67 nm). In the dipole approximation, photoelectron energy spectra (PES) are calculated by solving TDSE in a single active electron approximation with a Hamiltonian. The PES spectra are analyzed for different Debye lengths using the infinite surface flux method. A shift in the PES spectra is observed with the variation in Debye length, and a corresponding shift in the photoelectron momentum occurs as the Debye length changes.