I am not sure whether your code will execute successfully. To me, the code should look like this:
clc; close all; clear;
u=0.8:0.001:1.2;
v=1;x0=1;w0=1;w=0.44;T=2*pi/w;
axis([0.9 1.2 -0.8 1])
j = 1; % use a for loop if you want
fun = @(t,y) zjzdfun(t,y,u(j),x0,w0,v,w);
[t,y] = ode23(fun,[0 70*T],[4,4]);
plot(t,y)
function ydot=zjzdfun(t,y,u,x0,w0,v,w)
ydot=[y(2);
u*(x0^2-y(1)^2)*y(2)-w0^2*y(1)-v*cos(w*t)];
end
