A channel estimation method, comprising: obtaining (201; 901) a preamble in a signal packet sent by a first communications node, wherein the preamble comprises at least a first field and a second field, a subcarrier of each orthogonal frequency division multiplexing, OFDM, symbol of the first field is used to carry a first reference signal, the first reference signal is a determined signal that is known to both a second communications node and the first communications node, a subcarrier of each OFDM symbol of the second field is used to carry useful information, and the useful information is physical layer control information and/or data that is unknown to the second communications node, a quantity N of spatial flows are transmitted in parallel by means of multiple-input multiple-output, MIMO, the signal packet sent by the first communications node includes L OFDM symbols in total, and the first field and the second field respectively include L 1 and L 2 OFDM symbols, that is, L 1 + L 2 = L, wherein L ≤ N, L 1 = mK, L 2 = nK, and K ≥ 2, all subcarriers in each OFDM symbol of the first field and the second field sequentially correspond to different spatial flow groups, spatial flow groups corresponding to subcarriers of a same location in different OFDM symbols are different, and the spatial flow group comprises K spatial flows; in the first field, after undergoing orthogonal transform, the K spatial flows of each spatial flow group are sequentially transmitted by using subcarriers that are corresponding to the spatial flow group and that are of K OFDM symbols of the first field; and in the second field, after undergoing orthogonal transform, the K spatial flows of each spatial flow group are sequentially transmitted by using subcarriers that are corresponding to the spatial flow group and that are of K OFDM symbols of the second field; and obtaining, by using the first field and the second field in the preamble, a first channel estimate, of each spatial flow, on all subcarriers of a multiple-input multiple-output, MIMO, transmission frequency band. obtaining (201; 901) a preamble in a signal packet sent by a first communications node, wherein the preamble comprises at least a first field and a second field, a subcarrier of each orthogonal frequency division multiplexing, OFDM, symbol of the first field is used to carry a first reference signal, the first reference signal is a determined signal that is known to both a second communications node and the first communications node, a subcarrier of each OFDM symbol of the second field is used to carry useful information, and the useful information is physical layer control information and/or data that is unknown to the second communications node, a quantity N of spatial flows are transmitted in parallel by means of multiple-input multiple-output, MIMO, the signal packet sent by the first communications node includes L OFDM symbols in total, and the first field and the second field respectively include L 1 and L 2 OFDM symbols, that is, L 1 + L 2 = L, wherein L ≤ N, L 1 = mK, L 2 = nK, and K ≥ 2, all subcarriers in each OFDM symbol of the first field and the second field sequentially correspond to different spatial flow groups, spatial flow groups corresponding to subcarriers of a same location in different OFDM symbols are different, and the spatial flow group comprises K spatial flows; in the first field, after undergoing orthogonal transform, the K spatial flows of each spatial flow group are sequentially transmitted by using subcarriers that are corresponding to the spatial flow group and that are of K OFDM symbols of the first field; and in the second field, after undergoing orthogonal transform, the K spatial flows of each spatial flow group are sequentially transmitted by using subcarriers that are corresponding to the spatial flow group and that are of K OFDM symbols of the second field; and obtaining, by using the first field and the second field in the preamble, a first channel estimate, of each spatial flow, on all subcarriers of a multiple-input multiple-output, MIMO, transmission frequency band.