For the implementation of power market in China, medium- and long-term security checks are essential for bilateral transactions, of which the electricity quantity that constitutes the generation feasible region (GFR) is the target. However, uncertainties from load forecasting errors and transmission contingencies are threats to medium- and long-term electricity trading in terms of their influences on the GFR. In this paper, we present a graphic distortion pattern in a typical three-generator system using the Monte Carlo method and projection theory based on security constrained economic dispatch. The underlying potential risk to GFR from uncertainties is clearly visualized, and their impact characteristics are discussed. A case study on detailed GFR distortion was included to demonstrate the effectiveness of this visualization model. The result implies that a small uncertainty could distort the GFR to a remarkable extent and that different line-contingency precipitates disparate the GFR distortion patterns, thereby eliciting great emphasis on load forecasting and line reliability in electricity transactions.