Abstract: To reduce the error rate and packet loss rate of transmission data in embedded soft PLC system, a circle buffer of embedded soft PLC system was designed, mathematical model of the circle buffer was established. The state equation and the state transition probabilities of the circle buffer were analyzed, and its capacity and the formula of probability of being filled were also discussed. At last, the factors affecting the size of the buffer capacity under different conditions were obtained by simulating, and the relation of data buffer size and the difference between the input stream and output stream were concluded under the premise of no data loss: in the case of other conditions unchanged, the greater the difference between input stream rate and output stream rate, the greater the needed capacity for circle buffer.