1. A method for authenticating the identity of a handset user in a cloud-computing environment, wherein the handset is connected to a cloud server through a communication network, the cloud server stores a face sample image library corresponding to the user, and the method comprises: obtaining, by the handset, a login account and a password from the user; judging, by the handset, whether the login account and the password are correct; if the login account or the password is incorrect, refusing, by the handset, the user to access an operating system of the handset; if the login account and the password are correct, sending, by the handset, the login account and the password to the cloud server, wherein the login account and the password correspond to a face sample image library of the user stored on the cloud server; acquiring, by the handset, an input face image of the user; sending, by the handset, the input face image to the cloud server; authenticating, by the cloud server, the identity of the user according to the login account, the password and the input face image; judging, by the cloud server, whether the user is allowed to access the operating system of the handset; wherein the step of judging whether the user is allowed to access the operating system of the handset comprises: step A. determining, by the cloud server, according to the login account and the password, the face sample image library of the user corresponding to the login account and the password; step B. obtaining, by the cloud server, a face feature similarity value, FFSV, according to the input face image and the face sample image library; wherein the face feature similarity value represents the degree of similarity between the input face image and each face sample image in the face sample image library; step C. judging, by the cloud server, whether the face feature similarity value is larger than a preset threshold, wherein the preset threshold is obtained according to multiple first characteristic distances between each face sample image in the face sample image library; step D. if the face feature similarity value is not larger than the preset threshold, allowing, by the cloud server, the user to access the operating system of the handset; step E. if the face feature similarity value is larger than the preset threshold, calculating, by the cloud server, a first number and a second number, wherein the first number is the number of face sample images in the face sample image library corresponding to the first characteristic distances which are larger than the face feature similarity value, and the second number is the number of face sample images in the face sample image library corresponding to the first characteristic distances which are not larger than the face feature similarity value; judging, by the cloud server, whether the first number is larger than the second number; step F. if the first number is smaller than the second number, refusing the user to access the operating system of the handset; step G. if the first number is not smaller than the second number, allowing, by the cloud server, the user to access the operating system of the handset; and the step of B comprises: step B 1. obtaining, by the cloud server, a face region image from the input face image; step B2. calculating, by the cloud server, a first characteristic value of each face sample image in the face sample image library and a second characteristic value of the face region image; step B3. calculating, by the cloud server, a characteristic distance between the first characteristic value of each face sample image in the face sample image library and the second characteristic value of the face region image, to obtain multiple second characteristic distances; and determining, by the cloud server, the face feature similarity value, FFSV, according to the multiple second characteristic distances.