Blue energy conversion, where the chemical energy stored in salinity gradients can be converted into electricity with ion-selective nanochannel membranes, has considered to be one of the most promising renewable energies. Conventional understanding on this energy suggests that as to largely reduce the resistance, ultrashort channel membranes are required to gain high-energy output. To understand the channel-length-dependent blue energy conversion in detail, we engineered a series of highly ordered and uniform ~23.0 nm in diameter alumina nanochannel membranes with various lengths. Most anomalously, our experiments however show that for sufficiently short nanochannels, the shorter the channel length, regardless of surface charge nature, the smaller the generated power, violating the past understanding. The anomalous channel-length-dependent blue energy conversion is well supported by our rigorous model. The modeling reveals that ultrashort nanochannels will induce the significant ion concentration polarization effect, which appreciably undermines effective salinity ratio and ion selectivity in the nanochannel. If this effect dominates, the nanofluidic osmotic power turns into a decrease with decreasing channel length. Both the experimental and theoretical results reported consistently highlight the importance of osmotic ion transport especially in ultrashort nanochannels, and this finding shed light on the design of high-efficiency blue energy harvesters.