This event-related potential (ERP) study examined the effect of congruent emotional context on the processing of subsequent emotional words during discourse comprehension. We used two-sentence discourses as materials and constructed three types of discourse: one where both sentences conveyed a negative emotion (Negative-Negative), one where the first sentence conveyed a neutral emotion and the second a negative one (Neutral-Negative), and one where neither sentence conveyed a negative emotion (Neutral-Neutral). In all conditions, an identical target word was embedded in the second sentence that, combined with the local context, conveyed either a negative or a neutral emotion. Participants were asked to read for comprehension and rate the emotionality of each discourse. The ERP results showed that, when both sentences conveyed negative emotion, the target words induced a smaller N400 than when neither of the sentences conveyed a negative emotion. The target words also induced a larger late positivity when both sentences conveyed negative emotion than when either only the second sentence or neither sentence did. Our results indicate that congruent emotional context weakens semantic analysis and leads to more elaborated emotional evaluation in the later stage of discourse comprehension. These findings help clarify the effect of emotional context in language comprehension.