  The aim of this paper is to develop and analyze numerical schemes for
approximately solving the backward problem of subdiffusion equation involving a
fractional derivative in time with order $\alpha\in(0,1)$. After using
quasi-boundary value method to regularize the "mildly" ill-posed problem, we
propose a fully discrete scheme by applying finite element method (FEM) in
space and convolution quadrature (CQ) in time. We provide a thorough error
analysis of the resulting discrete system in both cases of smooth and nonsmooth
data. The analysis relies heavily on smoothing properties of (discrete)
solution operators, and nonstandard error estimate for the direct problem in
terms of problem data regularity. The theoretical results are useful to balance
discretization parameters, regularization parameter and noise level. Numerical
examples are presented to illustrate the theoretical results.
