The invention relates to instruction downlink encoding and decoding methods based on mud-pressure pulses. The instruction downlink encoding and decoding methods are characterized in that the coding method comprises the following steps of: controlling a pulse valve to be opened or closed according to the designated time sequence; generating four mud negative pressure pulses at the bottom of a well to finish the transmission of one rotary guiding downlink instruction; arranging two command zones and a check zone in the four negative pressure pulses; arranging and combining pulse time intervals in the two command zones to characterize the transmitted downlink command; and taking a pulse time interval in the check zone as a check downlink instruction. Corresponding to the coding method, the decoding method comprises the following steps of: detecting falling edges of the four mud negative pressure pulses at the bottom of the well by a well bottom mud pressure measuring device to acquire time parameters corresponding to the command zones and the check zone in actually-measured pulses; and then analyzing a downlink construction received by the well bottom by calculating and checking the correctness of the downlink instruction. The instruction downlink encoding and decoding methods disclosed by the invention are simple and feasible and more flexibly in setting; and the requirement of a rotary guiding drilling tool can be met and the transmitting work of various simple and complex instructions is finished.