Efficient utilization of energy is a core area of research in wireless sensor networks. Data compression methods to reduce the number of bits to be transmitted by the communication module will significantly reduce the energy requirement and increase the lifetime of the sensor node. This paper divides WSN applications into two categories, slowly-varying applications and non-slowly-varying applications, and indicates that the study of slowly-varying data compression algorithm is significance. Through analysis the probability distribution of monitored data, a simple lossless data compression algorithm namely Normal Distribution Encoding Algorithm specifically designed for slowly-varying data is proposed in this paper. Compared with the typical lossless compression algorithm, the proposed algorithm indicated better compression ratio and performance.