Abstract: In 2014, Yuan et al. proposed a new multivariate public key cryptosystem by combining "small field-big field" and "stepwise triangular" methods. The authors claimed that their scheme can be secure against rank attack, linearization equation attack and differential attack. Through analysis, we found that there are a lot of linearization equations satisfied by this scheme. We can transform it to an equivalent square encryption scheme by linearization equation method and then recover corresponding plaintext for any given cipheretext by differential attack. As to two recommended parameters, for given public key, the complexities of recovering plaintext are 2³³ and 2³⁵, respectively. The results above are further confirmed by computer experiments.