Matrix multiplication is a very basic computation task in many scientific algorithms. Recently Lei et al. proposed an interesting outsource protocol for matrix multiplication in cloud computing. Their proposal is very efficient, however we find that the proposal is not so secure from the view of cryptography. Concretely, the cloud can easily distinguish which matrix has been outsourced from two candidate matrixes. That is, their proposal does not satisfy the indistinguishable property under chosen plaintext attack. Finally we give an improved outsource protocol for matrix multiplication in cloud computing. © Springer Nature Switzerland AG 2019.