Is my approach secure for implementing a one-time token in web services call

Assuming that I have two systems (A & B) that i need to integrate with each other using web services,system Aoffers web services that retrive different types of documents (images,word doc,pdf, excel, auto cad, etc) ,so i am thinking of implementing the following security mechanism:-
system B will send a request containing a username and password to system A asking for the one time 64-char password.
System A will send a time synscrozied (based on current date & time) onetime password or token containing 64 characters to system B.
System B will call system A web services using the 64 characters password.
System A will check if the 64 -char password is valid.
system A will respond to system B request.
system A will removed the 64 password from the valid password (so incase system B or another system calls system A web services using the previously generated 64- ...