In today's market, mystery boxes (where the contents are uncertain) introduce uncertainty concerning the outcome, offering the consumer the chance to receive one of several possible foods. However, the role of uncertainty-of-outcome induced in food preference is unclear. This research investigated how uncertainty-of-outcome affects food preference for high- versus low-calorie food products, and its cognitive and neural mechanisms. Fifty-eight participants completed a binary-choice task, deciding between a certain option (a known food) and an uncertain option (an unknown food hidden in a mystery box), and rated the expected tastiness (subjective value) of each food. The participants exhibited a stronger preference for uncertain options in the low- (vs. high-) calorie case. Using the drift-diffusion model, the drift rate was higher in the low- (vs. high-) calorie condition. Moreover, a larger frontal N2 amplitude and smaller frontal P3 and LPP amplitudes were detected in the low- (vs. high-) calorie condition during the binary-choice task. Besides, frontal LPP amplitudes were negatively correlated with drift rate, suggesting that more cognitive effort was required to accumulate evidence to make a decision. Additionally, high-calorie foods elicited larger frontal alpha event-related desynchronization than low-calorie foods, thus suggesting participants require more evidence and effort in value comparison during the decision-making. This research highlights how uncertainty-of-outcome enhances the reward value of food, especially low-calorie foods, and thus impacts food preference, providing insights for developing marketing and public health strategies.