This paper analyzes the pricing of CSI300 stock index futures under a modified Cost-of-Carry framework. We discover a pricing bias in both long-term and short-term contracts which changes consistently over time in both average magnitude and volatility. We adopt a Brownian motion model with drift term to approximate the magnitude and volatility of the bias, and empirically analyze the factors that may affect this pattern. Generally, the bias is affected by market trading volume, investor sentiments, and futures related features. The study provides new evidence for the origin of mispricing in the Chinese stock index futures market and complements conclusions in previous literature.