7. Find the maximum value of the expression
$$
|| \cdots|| x_{1}-x_{2}\left|-x_{3}\right|-\cdots \mid-x_{1000}
$$

where $x_{1} ， x_{2}, \cdots, x_{1000}$ are different natural numbers from 1 to 1990.
7. Noting that when $x \geqslant 0, y \geqslant 0$, the following formulas hold:
$$
\begin{array}{l}
|x-y| \leqslant \max \{x, y\}, \\
\max \{\max \{x, y\}, z\}=\max \{x, y, z\},
\end{array}
$$

we can directly obtain
$$
\begin{array}{l}
\left.|| \cdots|| x_{1}-x_{2}\left|-x_{3}\right|-\cdots \mid-x_{\mathrm{n}}\right\} \\
\leqslant \max \left\{x_{1}, x_{2}, \cdots, x_{\mathrm{n}}\right\},
\end{array}
$$

where $n=1,2, \cdots, 1990$. Therefore, the value of the expression in question does not exceed $\max \left\{x_{1}, x_{2}, \cdots, x_{1990}\right\} = 1990$. However, since its value has the same parity as the sum
$$
\begin{array}{l}
x_{1}+x_{2}+\cdots+x_{1990} \\
=1+2+\cdots+1990 \\
=995 \times 1991
\end{array}
$$

the value of the expression cannot be 1990, so the maximum possible value is 1989. The following specific example
$$
\begin{array}{l}
|\cdots||| 2-4|-5|-3|-\cdots|-(4 k+2) \\
|-(4 k+4)|-(4 k+5)|-(4 k+3)| \\
\cdots|-1986|-1988|-1989|-1987 \mid \\
\quad-1990|-1|=1989
\end{array}
$$

demonstrates that the maximum value sought is 1989.