Abstract: On the basis of the Walsh sea clutter model and measured data of over the horizon radar(OTHR), a modified OTHR sea clutter model is proposed. Ocean current, vertical motion of the ionospere and the phase contamination which caused by nonstationarity of the ionospere are all considered in this proposed model. Using the modified sea clutter model, the effects derived from oceanic state and nonstationarity of the ionospere are deep analyzed based on blind zone on Doppler spectrum of ships. After comparing with the real sea clutter data, it shows that the modified OTHR sea clutter model is better than original Walsh model. The results will provide theoretical references for the optimal OTHR parameters and ionospheric channel selection.